She's not the most scintillating of characters, but in my opinion there's many far more boring characters than Maria in Corrie at the moment. She hasn't had any decent storylines for a long time which doesn't help.
I used to think she was good back when she was involved with Liam and then later Tony Gordon and also during her stillbirth storyline. I haven't seen anything (other than lack of storylines) since then to change my mind.
Put it this way: for me Maria is a middle of the road character who's had some very good moments in the past. She's not in the top tier of characters/actors in Corrie, but neither is she someone who I'd axe given half the chance (and there's quite a few of those).