To say that "nobody" liked Kid Goku is kind of a broad statement. Do you base this on Internet opinions, which is usually a vocal minority or do you have sale figures of the Kid Goku DLC to back this up? After all, sales ultimately tell us how successful a DLC is.
Calling him a half-ass seems like a personal opinion. He had few interactions with other characters but did have unique level 3 supers for which they created an entirely new Golden Ape model. Not many characters have a completely new model, just for a Super.They brought Baby, but no Omega Shenron, and Baby is honestly half-ass since he has almost no interesting inactions with other characters and almost everyone has generic quotes with him.
Being a DLC character doesn't say anything about it's popularity. If anything, a company would release more popular characters as DLC since people would be more willing to pay "extra" for those.Super Saiyan 4 Gogeta is stupidly popular, so him not being in the base game or at least the first wave of DLC and not coming out until the last DLC pack is actually really weird. Like, how the heck did Videl get in before him? Also keep this in mind, there was no GT characters at all in the base game and it took until DLC pack 2 to get GT Goku, while Super got two characters in the base game.
What people are you talking about? How large is the group of people that are online and live in Japan that supposedly say that GT is forgotten? What is the actual source?GT being closed to forgotten in Japan comes from people online who lives in Japan, a lot of Dokkan and Legends people who also dips into Dragon Ball Heroes merch. Outside of the Super Saiyan 4s and to a much lesser extent Omega, you don't find too much GT related stuff. There is also the Dragon Ball Super Card Game where 90% of anything related to GT is actually from Heroes. For the lack of better words, Heroes has been keeping GT alive for at least a decade.
This is also how I learned that the Japanese are honestly not that big on things past the Namek Saga.
While the show itself has ended long ago, it's popular enough to get a recent run in Saikyou Jump. If people would dislike the adaption, it would get yanked out of Jump before it could end it's run.