Seems completely legit to me - use stats if you're directly hitting someone to give a sense of strength and durability, but don't feel at all restricted by them. Koffing's a pretty slow Pokémon in theory, but I had one rocketing around on trails of gas and dodging hits by firing gas from the vents on the side of its body, so there's plenty of room to play it by ear as long as you're not trying to outright punch harder than you're supposed to.