Malevolent, yes. Outright evil, no, I don't think so. He did try to make his "subjects" happy, listening to their requests and granting them, and he chose to banish the adults rather than killing them or even harming them in any way (of course, if they had stayed trapped in that crappy dimension, sooner or later they would have starved to death or something; but I doubt Mordred planned that: I'm arguing that he was not really evil, not that he was not dumb).
He certainly deserved a harsh punishment, but, if you ask me, the kind of horrific punishment he got was way too much, I really don't think he deserved that. Death would have been a far kinder fate.