http://strawpoll.me/3509876

4020397

Princess Bellyestia.

It all really depends on which Celestia. Are we talking about the Celestia that is an almighty being that created Equestria, and made this form as her mortal avatar? Or is she the celestia that is the daughter of Queen Faustia?
Or some other Celestial?
If we are talking about canon Celestial, from the show, then my answer is yes, Princess Celestia does in fact have a belly button.