To start off, I love this picture! Pinkie Pie looks so happy and punky! I really enjoy your drawing style. Something about the way their faces are drawn really makes me love this as true, original art.
Spike looks as though he's not putting his soul into it. Scootaloo was screaming like a metal singer, and I just don't get that vibe from Spike. His expression make it feel like he's just singing in a regular voice. Unless that is what you intended, then its fine.
I'm having troubles discerning where Big Mac's body is. I can see his head, but as for the rest, I don't know whats going on.
Other then that, this drawing is Amazing and really draws your attention, knowing it was well thought out and carefully drawn.