In Mami's defense, she (as far as we know) is consistently forced to use her wish to save herself from dying. Then she finds out that in doing so, she's enslaved herself into becoming something worse than simply dying, becoming a monster that kills others. That is a fate completely opposed to her sense of justice, helping others as much as possible, so they'd never have to experience a fate like her own.
I can't say I'm really surprised she'd crack.
Her loneliness may be a side effect of what Homura has been doing. Mami gets to be a puella magi for some time, but Madoka joins her...just because. Also fairly early on from what we can infer from this episode. Homura is keeping that from happening, which left Mami all by herself.
note: I loved the scene with Madoka hugging Homura and drumming her feet after Homura's debut battle. So cute~! That said, a peppy Madoka is a bit...weird. I'm so used to her being filled with doubt and being introspective.