The problem with the bunny is that it doesn't add anything fun.

It's a punishing failure mode for a basic part of the kit, and the punishment is not trivial—you lose the mudra charges (already a scarce resource) and get locked out of ninjutsu for several seconds. Mudras aren't difficult or skill intensive nor do they boost NIN dps past other classes—there's no reason to add a punishing failure mode. I take it that the bunny makes people feel elite for successfully casting ninjutsu—I like the ninjutsu system a lot but don't see it as significantly harder than 1-2-3 combos, so I don't get that feeling myself. Imagine if moving while casting a spell dropped your mana to 0 and prevented all regen for 6 seconds—some people (BLMs, looking at you) might find that fun, but I'm inclined to think that most would find it needlessly punishing.

At the least, I think that mudra should be greyed out once pushed until a ninjutsu is cast or 6 seconds pass. (I would go further and refund mudra if there's no ninjutsu cast after 6 seconds.)

The punishment doesn't fit the crime!