I'm pretty sure that
will have decent matchups across the board. He seems to do well against some characters that do not have the best approach options, but he might struggle against rushdown and small characters along with characters that can wall others out pretty effectively through projectiles. It might be difficult, but our King will have a good comeback factor by the fact that he'll win trades in almost any situation and he kills pretty early. I think you just need to win neutral a few times compared to other characters.
Also,
does the Blunderbuss only have a single throw angle when the cannonball or a fighter gets sucked in? From my knowledge, you can only throw it straight diagonally up to the way K Rool is facing, or you throw it in the opposite direction.