I find Polonius a bit foolish. He was a a busy-body, which means that he was into everyone's business, meddling in affairs, and projecting his own opinions on issues all of the time. He gives his son, Laertes, a bunch of long-winded advice about life and living successfully, which was delivered at such length that it was a bit tiresome. Foolish old men like to think that they know more than everyone else, and are constantly dispensing advice, whether it is sought after or not.
Polonius is also constantly getting into the king and queen's business, in an almost "suck-up" sort of way. He takes information given to him by Ophelia, about Hamlet acting strange, and goes and blabs it to the king and queen. Then, he manipulates a meeting between Ophelia and Hamlet to please the king and queen, and help them in their quest to determine what is wrong with Hamlet. How would you like it if your dad found out that your boyfriend or girlfriend dumped you, then went to their parents and set up a secret meeting with you and the ex-boy/girlfriend just to see how everyone would act? Very meddling and embarrassing indeed. So, in that, I feel that Polonius was a bit foolish. In the name of sucking up to the queen and king, and of being a bit of a gossip, Polonius kind-of throws his daughter at the whim of royalty. I find that foolish and rude.
Then, Polonius foolishly hides himself behind a curtain while the queen and Hamlet have a private conversation. Yes, that's a bright idea, isn't it? How annoying, and why is it his business anyway? Granted, he didn't deserve getting stabbed for it, but, if he hadn't been foolishly putting his nose into other people's business, he would have never been there in the first place. So, for those reasons, I find Polonius a foolish old man who talks too much, meddles too much, and contributes to his own demise.