I know it is the most busted item. It nullifies most of the other power ups. That is why I recently updated the generation of the sequence so it is the most difficult one to find.
In previous versions you had to find all the coins to find mount hop. This no longer necessary. Basically Mount Hop is located above all the rooms. So now the "ropegun" is actually the "key".
Also finding Mount Hop doesn't wipe your data anymore. So you can still collect all coins after that.