What is comparative and superlative degree of munch?

Oh, honey, it's simple. The comparative degree of "munch" is "munchier," and the superlative degree is "munchiest." So, if you're comparing how much someone can munch, you'd say they're "munchier" than someone else, and if they're the ultimate muncher, they're the "munchiest." Easy peasy, lemon squeezy!