Sorry, but Martinez would've been a way bigger catastrophe (for Martinez) and a way less logical option. Why would someone like Mercedes align herself with Eva Marie, who claims the matches that Mercedes wins for herself?
Piper Niven is the perfect fit for what they are doing. Piper works as an obedient monste for Eva, who doesn't care at all about Piper and can claim the victories for herself, while holding her jiggly tits in the camera... couple months down the road, Piper probably turns face, because she overheard that Eva laughed about her body, or insulted her behind her back or some shit like that, we get maybe a B-PPV match and afterwards you won't see Piper much anymore.
The whole thing isn't about elevating the person besides Eva to the next level, it's just about drawing in Evas social media numbers.