I don't think a goblin HAS to be evil. Like, I don't think it's literally baked into their DNA. But a goblin who is raised among normal goblin society is almost certainly going to be evil. If some good and kind people raised a goblin in a loving, nurturing, moral environment, they could probably be just as good as anyone else.

But the goblins we see in BG3 are absolute shitlords, even the kids.