Oh boy what an interesting thing to discuss, Inhumans. What a fascinating show, and Medusa is definitely part of what makes it so interesting:
When we first meet Medusa, there is nothing to like about her. Both her and Black Bolt are shown to lead this society and enforce a rigid caste system based (essentially) on your DNA. How on Earth are we supposed to like these characters? As Maximus leads his coup and fights for equality, we have to keep reminding ourselves that this is the bad guy, because there's nothing redeemable about any of the good guys.
But as the show progresses, Medusa doesn't change much. She meets Louise, who is easily one of the best parts of the show, if not the best. The show cleverly uses Louise to trick us into caring about Medusa (as it does with all of its Earth-dwelling characters), but we never get insight into Medusa. Sure, we get the flashback with a young Black Bolt and Medusa, but that doesn't help establish any character, she still believes in the ruling. However, we grow to care about Louise through her love of space, her immediate protectiveness of Medusa, and her overall personality, so by proxy we care about what's happening to Medusa.
I remember the specific moment where I went "Sh*t, I actually care about this." That moment was when the Inhumans were reunited (except Crystal) and about to return to Attilan, after Gorgon died. The characters were talking, and Louise mentioned possibly ending the caste system. Medusa and Black Bolt did absolutely no growth as characters, nothing about them changed. But Medusa still listens to Louise, and the audience still gets to feel proud of her as she even learns to stand up to Black Bolt (slightly out of nowhere, with nothing beforehand telling use this was a problem, but still). Medusa grows and promises to change the way Attilan is run as it's being destroyed, and suddenly we care about her.
This is the case with all of the Attilan-dwelling Inhumans in the show, and it was a large part of Inhumans' problem. I wouldn't say that Medusa is the best case study of that (for that I'd go to Karnak, Gorgon, and Black Bolt), but how rushed her story is to the point where I can't make this much longer than it is, despite her being around for the whole season, makes her a good place to talk about it.
The actress was nothing special, but she also was necessarily bad. It was a mediocre portrayal.