Ah, yes, I'm pretty sure Mac and Cheeri were left as good friends. This is Marble Pie, Pinkie's younger sister. Imagine Fluttershy as Pinkie's sister. An interesting comparison, with Pinkie and Fluttershy sharing a voice actress. She and Mac share a few cute scenes together in the newest episode. Naturally this is where everyone took it.
I should point out I don't endorse or support either ship in any way. I don't disapprove either.