I have noticed that many people tend to underestimate characters because of their inherent silliness. I myself used to do this all the time, for example I used to think the Flash rogues were worthless.
However even seemingly goofy or stupid characters can be legitimately powerful.
Take Toyman for example.
At first you think "This guy's gimmick is making exploding dolls, how is he supposed to be a threat to Superman?"
Then you realize he's one of the most brilliant robot scientists on earth, and has created things like nanotech, etc. Also he usually doesn't directly confront Superman but instead uses plans to distract him by going after civilians.
Another good example is Stilt-Man.
He's a guy who walks around on stilts, ridiculous right?
It turns out his stilts are actually a form of powered exoskeleton that gives him super strength, and he has an arsenal of weapons like knockout gas, electrical shocks, laser guns, etc.
What other characters seem silly at first but are actually legitimately dangerous?
Charles Xavier. Try explaining to someone that he's actually really powerful because of his brain and the company he keeps and people don't seem to get it.
Not so much in the early days, but nowadays. People always remark on how they forget he was so badass, fast and one of Marvel's most formidable combatants. He's become more of an annoyance to people until they realise he's a legit threat.
Stilt-Man never posed any kind of threat to anyone but himself.
I can come up with a list of grossly underestimated characters pretty easily though:
Squirrel Girl
Bullseye
Mr. Fear/Larry Cranston
Purple Man
Mutant Zero/Typhoid
Crossbones
Living Laser
Black Tarantula
Sienna Blaze
__________________ Land of the free, home of the brave...
Do you think we will ever be saved?
In this land of dreams find myself sober...
Wonder when will it'll all be over...
Living in a void when the void grows colder...
Wonder when it'll all be over?
Will you be laughing when it's over?