The thing is that Pikachu is halfway between an animal and a stuffed toy. Its clearly not real, but those are both things that can and do exist in the real world so its not glaringly awful.
Sonic is a CGI abomination that this weird, cartoony thing that doesn't look like it belongs in the real world at all. He isn't animal like enough to pass as a creature, and his face is all sorts of weird. And then you get the weird details on top of that, like the fact that his hair is apparently about as thick around as wires, which just adds to the "fake and probably made of plastic" aesthetic.
Making a 'live action' sonic was always probably a doomed venture, and maybe this really is the best they could do. But that doesn't make it good.