mold strikes me as more grass-like than bug-like. Poison/Rock or even Water/Rock would make more sense too.
zygarde barely looks like a dragon or a ground type, I don't really get why it's either. I suppose dragon is somewhat excusable as it resembles dragons more than it does most other types, but ground is just dumb. The combo is dull at this point, with Flygon and also Garchomp sharing the same typing, not to mention similarish stats. Dragon/Bug, Poison/Steel, etc would have made more sense as well as being more interesting.
gyarados being water/flying never made much sense. I get that it levitates, but so do most fish pokemon when brought out of water, so why is gyarados flying while they're not. I bet it was water/dragon originally, but they retyped it so dragon would be exclusive to that stupid fat yellow pseudo. And no, I don't feel the same way about charizard because charizard actually looks like a flying-type.
no idea how mr. mime is a fairy while chimecho and co. aren't