Hello! ^^
I’ve been actively using UTAU for a couple months, so I'm not an expert, but if you’d like, I’m willing to give suggestions. ^^
The tuning is pretty good, and in some parts it’s pretty fluid; in others, it sounds a bit more robotic, but I think it’s because she’s a CV voicebank. I like how she sounds pretty upbeat during the song!
As for the mixing, I feel like the music drains out the voice a LOT of the time, or is at the same level as the vocals. During the first chorus, it sounded like the music and vocals were competing, and the music stood out more than the voice itself. It just sounds a bit distracting, at least to me. (I believe a lot of it has to do with the instrumentals, though)
I’m not sure what program you use for mixing, but if they offer any special effects for music (Adobe Audition has one, and it makes the music subtle enough that it won’t overpower the vocals), I encourage using them! If not, maybe reduce the music a bit. ^^