She was seen in "Positively Pink" practicing with the cymbols (of which you can see here overlaid on top) but then she and with many other members of Pinkie Pie's band are replaced by new band members when the performance starts. Her card's info says:
RB11-EN051:
My Little Pony Mish Mash Mee Incarnate
LIGHT / Pony / Level 2 / ATK 600 / DEF 400
You can Tribute this card; add 1 "My Little Pony Mish Mash Mee" from your Deck to your hand. Once per turn: You can target 1 Level 2 "My Little Pony" monster; change its battle position.
Rainbow Rare