I'm a rather plump girl (I'm still skinner than the girl in the game ) and I think the girl is very cute !
Yeah, she's drawn cute. Really, the translation of this title is a little off. In Japanese it's (ちょうど良いぽっちゃりブス)ぽっちゃり幼馴染と爛(ただ)れたセフレ関係になった僕. That's '(Nicely Chubby Homely [girl]) I Got Into a Sexfriend Relationship With a Chubby Childhood Friend'
Busu (ブス) is the key word here. It's like 'jimi' and can mean homely, plain, and sometimes get used by kids to call someone ugly just because it sounds bad ('BUSU!'), but it really is more 'not good looking' and plain rather than actively ugly. If they really wanted to say she was repulsively ugly there are better words like minikui, migurushii, and osoroshi.
So just substitute 'plain' wherever you see 'ugly'.