You might think this joke sucks, but the title says otherwise
I found this joke posted in reddit. The humor in this joke is a pun. The title of the post was the word "Otherwise", so literally the title of the joke undeniably said "Otherwise". That being said, "x says otherwise" means that if you were to ask x, they would tell you something besides what has been said before, so saying the title says otherwise is like saying that the title is a counterargument to the statement that this joke sucks.
The two meanings of the joke are why this is funny.