Would depend what type of tissue was destroyed. Muscles don't grow back, nor do many other types of tissue. If the damage is cosmetic the fish can grow back a fully functional mouth though. It's kind of like the difference between skinning the tip of your finger and cutting off the tip of your finger. In the one case you've damaged regenerative skin tissues only and they grow back fine. In the other, you've completely destroyed the tip of the finger and while the skin tissue will regrow everything that was within won't so your finger will be shorter than it was before, even though it has regrown skin.