I don't think she's that bad, she probably knows her limit as a actress and it helps she's a beautiful woman to boot. I think she's the go to woman for the girlfriend and wife roles in blockbusters or bigger budget films, she isn't going to cost much and is a professional and probably doesn't complain about how big her part is. She's got a charisma to her and she seems like a good person off set, that's why she gets offered so many jobs. That's why you see her in "Every" movie as she does more bigger budget films then indies.
She's a good comedian, one of the reasons she's got work on Spy, Neighbors, Bridesmaids and is solid in most blockbusters she's done like X-Men. She's got some good work on her C.V, Sunshine (2007), Marie Antoinette (2005), underrated sequel 28 Weeks Later (2007), Two Hands (1999) and Adam (2009). And some of her comedy work is very good especially Bridesmaids and Spy.
I think she does stuff to pay the bills and probably doesn't take enough chances to challenge herself but not every acting talent will do that.