Was Goku really a bad parent?

Goku is not a bad parent at all. Everything he did was to ensure that his family was protected at all costs. Before Raditz appeared, Goku was there for every waking moment Gohan was up, and showed him around Paozu and such. But alas, he died and was forced to defeat the saiyans and Frieza. Even upon dying again during the cell games, Goku felt that it was his fault that so many suffered due to Dr. Gero's grudge against him. So he felt compelled to stay away so that his enemies wouldn't target the Earth once more. Very noble despite the sacrifices that were ensured.

Goku has also never hit his kids once (despite that one time when he punch Gohan to prevent him from committing suicide by challenging Imperfect Cell) and will yell at them to save them from greater harm (at Gohan upon becoming a SSJ, at Goten to man up upon hearing his brother's supposed death, etc). There is a reason why Chichi, Gohan, and Goten love their father / husband very much, and that must be included before making the assumption that Goku is a lousy father. If anything, Paragus is the shittiest by far!

Overall, I would say Goku is a decent parent. I only say decent because I think by the end Toriyama now sees Goku as inherently selfish that he most likely thinks Goku is a bad parent. Key point, in the manga of Super has Goku act all pissy at Gohan choosing a work event over coming to the tournament, while I heard the anime had Goku be understanding about it and his son's choice not to be a fighter.

I think the bad father stuff comes from Cell arc. Goku screwed up so bad that it made us wonder if he knew his son at all (which Piccolo called him out on), didn't prepare him to face Cell (he kept Gohan in the dark instead of telling Gohan his plan and working with him unlock his power through anger during those nine days), and while he had good intentions by choosing to stay dead it can come off that he wanted to stay in Otherworld at the promise of new fighters and figured Gohan could handle things, forgetting that Gohan in the manga is only about 9 or 10 in Cell Games, thanks to how Toriyama later writes Goku (leaving to train Uub for a good fight more than wanting him to be a successor, which Vegeta suspects). All of that together is what started makes us wonder if Goku is actually a good father when before that most of the fandom would have agreed he was one. The final chapters don't do him any favors either considering how he ditched his family when he can just as easily as train Uub from home. He forced Goten to train.

Keep in mind Trunks and Goten are deemed lazy by their fathers, those chapters showed Goku hadn't kept in touch with anyone for years, because he spent most of his time training. That means Trunks and Goten wanting to get out of training can easily come off as their dads trying to force them to train as much as they do (which is hours on end and leaves them little time to do anything else or in Goku's case not see his friends). Goten and Trunks come off as less lazy and more of people who want to do more with their lives then spend every waking moment of it training as their fathers seem to be attempting to do. That potentially makes Goku possibly a worse father to Goten, since he at least accepted Gohan doesn't want to be Goku 2.0.

I can easily see why Goku is looked upon as a bad father with how Toriyama started writing him in the end and to say nothing else about how he sees Goku now. Really I would say Goku is a good example of a real parent. He has flaws and doesn't always get his children, but loves them, has their backs, and is very encouraging and affectionate toward them (which is why I don't buy Toriyama's Goku sees his family as friends and can't understand them as wife and children, god he makes Goku out to be worse than I think he ever was in the manga). He screwed up with Cell incident with Gohan, but upon realizing it he through away his honor, told Krillin to give him a senzu, and was going to go down there to save his son, before 16 jumped in. When he teleported Cell away to save the Earth he didn't blame Gohan for his carelessness. He encouraged his son and assured him this was something he would do for all of them. He even comforted Gohan about not blaming himself for his death and stayed with him every step of the way when he fought Cell. When he first met Goten he was again affectionate toward him and recognized Goten seemed hesitant about him since this was their first meting. Goku can be a good father and at one was one. He's not perfect, but I really disagree with Toriyama's image of him, if only because he seems to be asking us to ignore how Goku used to be written back in the heyday and all the character development he made to fit more in line with how he sees Goku now.

I think the "Goku is a bad parent thing" is mainly a TFS-inspired idea and it'll probably get annoying/discredited with the fan base within a few years.

From my limited Western perspective, Goku is a great dad. There's some obvious areas where he scores high--affection, kindness, stuff like that--but there's some less obvious ones, too. He's spending time with Gohan up until Raditz arrives, goes to rescue Gohan, spends a few years with him before the Androids arrive, and spends a year with him in the HTC. Once Gohan's born, I don't think Goku ever goes off to train in-canon without taking Gohan along. Also, when Goku came back for the World Tournament, he sought out Goten (who he'd never met!) and spent time with him. And when we first see him at EoZ (at least in the anime), he's with Goten again. Contrast to Battle of Gods, which begins with Goku training on King Kai's planet like a bum.

What else should be said is the difference in confidence shown in Gohan by Goku compared to Chi-Chi. Chi-Chi is often telling Gohan not to do things, while Goku is often allowing Gohan to do things. This sort of behavior is typical for many father/mother/child relationships (Mother is more protective, Father more reckless) but it should be pointed out that Goku legitimately trusts Gohan--to go to Namek, to fight Cell, to go fight Dabura and Babidi alone, to go kill Buu. He was also confident that Goten and Trunks (who he'd only met once!) could beat Majin Buu. This trust in his sons isn't always fulfilled, but it's there. I'm not certain if Vegeta has ever put a similar level of trust into Trunks.

If Goku has a flaw, it's that he failed to understand or take interest in anything Gohan liked that they did not have in common. My interpretation of Gohan is that he trained and learned to fight because he loved his dad. He loved to be with his dad, so he loved training because he got to be with his dad, not because of an inherent love for training and fighting like what Goku had. Gohan's the sort of boy that if Goku had been obsessed with fishing, or dancing Ginyu Force-style, or knitting sweaters, he would've loved to do all those things because he was with his dad. Goku never asked Gohan what he was learning about, or what he was researching, or what he wanted to be when he grew up. Goku never seemed to appreciate that it wasn't just Chi-Chi who wanted Gohan to become a scholar--Gohan actually did, too. As a parent, even if you don't give a flying fig about what your kids like, it's good to show interest in those things. Goku loved fighting, thought it was cool that Gohan would fight with him, and didn't really pay attention to anything outside of that. Allowing Gohan to be a scholar isn't quite the same as acknowledging Gohan wants to be a scholar.

Unfortunately, the 15 years of domestic life Goku had with his family in DBZ are never shown to us. It's quite possible he was the perfect dad in peacetime (which he probably was) and we just never got to see it. He sure did a good job with Pan at EoZ.