Itty bitty suggestion. Since there's no cool-down after you dismount to use a skill anyway, make it to where you are automatically dismounted and it uses the skill. I am always for convenience of the player, any small or seemingly insignificant thing can be tweaked to make life more pleasant for all involved. I believe this to be one of those seemingly insignificant things. EDIT: After further observation, make it to where pressing the "sit" button will toggle pressing down on the mount.
I would actually prefer it the way it is right now. I kind of instinctively press my skills sometimes and I like being on a mount to prevent that. Anecdote aside, it wasn't (I don't know if it is in GMS) a feature that the game had, and I don't think it's worth being changed, even if it were to be quick.