You can always ask, but if you plan on charging for usage, it might be a bad idea. Speaking from my personal perspective, if I used an app that asked for a donation, I might give something, but if a month later they started charging me, then I would resent the fact that I had been hit up for a donation earlier.
If, however, the intent is to NEVER charge the users based on their current usage (which will always be available) and you ask for a donation to help offset your expenses until some other revenue model picks up, then I don't think there would be a problem.
Even if you can ask, imho, donations are kind of pointless unless you have wikipedia scale level of traffic. People simply do not donate.