For the most part, no. I've seen no difference in performance or battery usage for the themes I've installed on my phone. There are a couple themes (Slanted Lock comes to mind) that does require the weather data on it to be refreshed which uses data and battery power to do so. Depending on your frequency of updates, it can affect your battery however not the phone's performance per se.
If you use vWallpaper then there will definitely be some battery drainage due to the animated wallpapers.
But like I said, there shouldn't be any adverse affect on your phone simply by installing a theme.