Yes it's an ultimate skin.
In concept, the skin being an ultimate skin is really silly; sona's not the most played champion out there, and even if she was, she's a support, hardly worth an ultimate skin to most people. When it was initially mentioned, of course everybody took it differently because of no real information of how it would be. Everyone just assumed the whole PF ez / SG udyr thing: Animations, Model, Effects, Icon(s), Banner. If that was the case, I honestly wouldn't want to have her as the one and only ultimate skin that we'll get for season 5.
The instant that it was revealed that her skin would change the default music for everybody on her team, as well as altering it whenever her abilities were triggered, on top of the fact that she has three completely different and epic soundtracks (and looks badass to add on to that), everybody took a huge 180. In my opinion this is the single greatest skin riot's made, and I honestly am curious how they'll actually top it.
TLDR: Everybody thought it was going to be bad in concept, but riot executed it more than flawlessly, and everyone loves it.