I am a Reservist and my cousin graduates this weekend. Is it appropriate to wear my dress blues?

Graduation seems like a fine place to wear it, just make sure everything is correct.

I hope you mean ASUs... but yes, that would be a more than appropriate occasion to wear your uniform. Just make sure that it is put together perfectly because a DS will not care that you aren't a basic trainee if you are jacked up.

This is questionable as an occasion for wear, and I would advise against it as you would likely be the only one there in uniform and the event is not of a military nature.
As for CAN you? I would say the right answer is to get authorization from your Chain-of-Command.
Pride in service is a good thing, but in my opinion this is not a good venue to show it.
This is the graduates' show.