Monotony does not a good game make, in any situation. In a brawling game, it's practically a death knell, a sign of laziness in development and a lack of innovation. And it's one of the key things that kills Mutant Melee before it can even get started.
While a fleshed out narrative for each character would be a bit far reaching, what the Adventure mode amounts to is little more than a series of mini-games that are repeated ad nauseam for under an hour per character.
What gets under your skin is that most of the combos are unblockable, and once you are caught in string of moves, you must wait until the move set is over before you can block or counter attack. This can be frustrating on both A.I. and human battles.