Do you all think Paps was sitting idly for 30 years? Even in the 6 years between RotJ and DE, Palpatine muses that he had grown notably stronger in the Force.
That said, he may not have been keeping his lightsaber skills as refined as they could be. He claims in Rise of Darth Vader that he had only put use into the Jedi weapons to mock his adversaries, and in fact had no need to continue using them; it's possible that between RotS and RotJ, he's let some of his technical skill slip. Though that too is only speculation, my own conjecture.