I think you could play with the proportions too. Bowser looks kind of cartoony...which is not a problem, but he seems like a character that would benefit from a more looming, ominous appearance in that style.
You may also want to desaturate it a little. The yellow is very bright on bowser. Asgore doesn't have any yellow features because it is fair to assume he is more orange, but you can tell his sprite is not bright and colourful. The green is probably ok, but the red and yellow could be toned down.
You may also want to desaturate it a little. The yellow is very bright on bowser. Asgore doesn't have any yellow features because it is fair to assume he is more orange, but you can tell his sprite is not bright and colourful. The green is probably ok, but the red and yellow could be toned down.