Not really. It's in NU, which is quite a difference from OU. Smogon doesn't have an OU analysis for it either. It's easily killed by Pokemon with a Steel move, of which there are many in OU, due to a 4x weakness to it. Corviknight, Excadrill, Ferrothorn, Magearna, Melmetal, and others greatly threaten Diancie. There are quite a few Pokemon better than it, like Tyranitar, Clefable, Magearna, and the Tapus. Tyranitar and Magearna have more offensive prowess than Diancie, Clefable and Tapu Fini are better walls, and the Tapu Koko and Lele are just better choices in OU. Diancie's certainly usable in the lower tiers, but it isn't very good at all in OU.
Hope I helped!