I don't think Marcy was ever a bad friend. Sasha was just her more dominating "friend", and so that made the three girls' relationship toxic. I definitely cannot see Marcy being a bad friend when it was just her and Anne. She did that one really dumb thing, yes, but that was one time. She was a bad friend for doing that, but until that point she seems like she was a pretty good friend. She clearly cared very much about Anne, despite being clumsy and oblivious. That's probably why Sprig was the first real genuine friendship she had. But Marcy was not a toxic friend like Sasha. She could never be.

And yeah, Sasha is evil, there's no two ways about that. She tried to murder Sprig and Hop Pop and usurp the throne of Amphibia, all after and while having been bullying and manipulating Anne into line her whole life. If that's not evil, I don't know what is. Yes, she's working on redeeming herself now, and that's great, but you can't say she wasn't evil before and during the early seasons.