i suppose goro probably just sticks around and plays along in maruki's reality because he has nowhere else to go. he does seem aware of what's going on.
and he probably does secretly like it to some extent because he gets to be joker's friend.
he may act like he completely hates the idea but he also acted like he completely hated joker and shido so i can't really take it at face value when he says he is completely against something.
and joker (through the pts) did acknowledge him for who he really is and hold him to his deal for their rematch so joker is the only one goro has to stick around for.

for whatever reason goro didn't/couldn't fight against maruki so his world stays. so in this event taking everything into account, where else would goro go in maruki's reality besides stick around joker? it's the only place he even sorta "belongs" and that's why he sticks around and plays along.