>>280422676
I would argue that the reason Saitama views him as wearing a costume is he KNOWS he's human, and he doesn't really buy the idea of just turning into a monster.
Garou was becoming a monster physically, but not mentally. He was still acting like a human, and Saitama was watching him beat up the S class heroes and observing how he was acting - like a human. Considering how many odd looking monsters there are, the fact that he specifically mentioned this as being a costume is striking, not because its less monstrous than others. There's a few monsters that have humanlike appearance, like the Clown (base form), Triple Tongues, Do-S, Devil Long Hair, Pureblood, Royal Ripper, etc.
To me, it was more Saitama seeing through his monster facade (personality), not his looks. Calling it a cheap costume is just an extension of that, because when you know someone is a human, then any monster appearance looks like makeup
