Yeah, I agree that she wasn't developed properly. It was a lot to throw two new main characters in our face in season 7. Her whole thing coming back was her divorce from David, which I don't think many people understood or thought she handled well. Other than that we knew she was a photographer. But after her and Clay got together, that pretty much defined both those characters. Which is sad, because then we didn't really get the chance to love them as characters or real assets to the show. Each had a few legit storylines in season 7 (Clay/Sara, Lydia dying), but after that Quinn was pretty pointless outside of being with Clay. She provided comedy, babysitting, and a half naked girl for the show.
Given it was an uphill battle at the time she joined the show, but they could've done more to develop her character. A lot of people appreciated her character the most come season 9, when Shantel's acting had improved a lot and Quinn was playing the selfless, supporting role.