Is funniest common noun?
No, the word 'funniest' is not a noun at all. The word 'funniest' is the superlative form of the adjective funny (funnier, funniest). Example: That is the funniest story the I've heard for why you don't have your homework. The the word funny (funnier, funniest) is the adjective form of the noun fun, a common noun; a general word for any fun of any kind.
Why does the word funny not go with most or more?
It is incorrect grammer. Instead of using "most funny" or "more funny" we have the words "funnier" and "funniest" Here are a few examples you can compare. "He is even more funny than you!" "He is funnier than you!" "She is the most funny of them all!" "She is the funniest of them all!" Using the words "funnier" and "funniest" reads better in sentences and also sounds considerably better.