Not stupid? I agree. But not evil? I don't know about that. Or do you think that he was just deluded?
It's a difficult question, mostly because "evil" is notoriously difficult to pin down. The first thing we have to realize is that nobody has ever done evil for the sake of doing evil, at any time in history - everyone is always firmly convinced that their actions are serving some good cause, no matter how objectively evil we all might think they are. Even Hitler thought this way - as objectively evil as his actions were, he truly believed that he was serving some higher good to do what he did.
This is not meant to excuse Hitler, of course, or to compare Wellstone to Hitler, really. But I think Wellstone was like every other person that has ever walked the face of the earth - he truly believed that he was acting for the greater good in doing what he did, and promoting the policies that he did.
Of course, being a clever person, you will probably immediately spot that a strict observance of that principle would lead to the conclusion that no one is ever really evil, even Hitler. And that's not quite where I want to go. It's something of a judgment call as to where "wrong" ends and "evil" begins, but one can make an argument that Wellstone's policies promoted more good than harm - it's not a very good argument, which is why I think it's completely wrong, but you can make a reasoned argument. On the other hand, it is rather more difficult to reasonably argue that more good than harm is brought about by the eradication of all Jews, for example.
Even if we find Wellstone's policies to be harmful on balance, labeling our political opponents as "evil" is someplace I am loathe to go. They've been playing that game for a long time, and I'm not in a hurry to imitate the left in that particular tactic. Hold the moral high ground, and patiently explain why your opponent is not evil, not cruel - just wrong. Let them do the fearmongering and demagoguing. If beating the left means using the same horrible tactics they've grown so fond of, it's not a victory worth having ;)