Maybe, maybe not. Moya also isn't a natural right hander and still his serve was always a very good one. So are many others.
There are always exceptions.
I remember an interview where Rafa or someone in his camp said that Toni thought his lefty serve was no worse than his natural serve when he was much younger. I'm not sure, but I don't think that's good at all...
Who knows? Serving right handed he may have had a better serve, a worse serve, or the exact same serve. Of course, if he was playing right handed who's to say he'd even be in a position where we would know who he is to speculate about such things.