If I said that sharks had flippers, you might call me stupid. Sharks have fins, not flippers. But what if I said that sharks had propellers? You'd either think I had mental problems, or you would chuckle.
Subtlety in writing is tricky. If people aren't familiar with you or your style, it is very easy for subtle humor to be taken as straightforward, inaccurate statements.To avoid that confusion, the easy way is to be spectacularly wrong.
Don't go halfway; be absurd. If you want your character to sound dumb, make him ask if zebras are those horses with the horns. Have an inept chef make a fruit pie with whole fruits in them (as in cores and stems and rinds).
Sarcasm is the worst of them all. When a character says, "sure, I'd love go to the movies with you", there is no way of telling if that is sincere or sarcastic (again, unless you are very in tune with the character). To make us sure that she is being sarcastic, have her say, "Sure, I'd love to go to the movies with you. After that, we could try cyanide burgers or a refreshing dip in the shark tank." Is it ridiculous? Most definitely. Is the intent thoroughly clear? Most definitely.
Even when our stories are meant to be realistic, it is larger than life. Don't be afraid of it. Embrace it. Say some ridiculous things. If you're going to be wrong, be spectacularly wrong.