In the "Pre-Crisis" 2-part story "Whatever Happened to the Man of Tomorrow?", Mr. Mxyzptlk revealed himself to be more than "a funny little man in a derby hat".
When he first appeared, his clothes were darker, his hair was straggly, and his face a mien about it that bespoke evil.
He tossed his dark purple derby to the side and transformed into his "true self", and Lois Lane, who had been narrating the story for a reporter in the not-too-distant future, looked at IT, she said "My head hurt."
When he saw that Superman was about to chuck him into the Phantom Zone, he spoke his name in reverse, hoping to make a getaway.
He was half right, because only half of him got away. The other half went into the Zone, effectively killing him. Superman, who had sworn an oath against taking a life, stepped into a room filled with Gold Kryptonite, and that was the last he was seen.
And no, you are not the only one who remembers that one of Superman's sole weaknesses is magic, but I would bet that I am one of the very few who remembers a "Post-Crisis" moment, I believe, where it is supposed that Mxykins is also another character that is Impossible to live with. ;-D