That's true, if damage is extensive enough it may not grow back. Since the description was nipped or torn fins, it's a good chance it will heal.
I can only speak from experience of about 4 fish having damaged fins. The reasons ranged from getting picked on or beat up to having flukes. All of them healed after giving them a better environment.
As for diseases, my first guess would be parasitic for torn or tattered fins. I would guess if it was bacterial, there would be some redness or sores you would notice.
But, if was a disease I would look for other signs first. The only disease I know of really that shows moth eaten fins as the only sign you can see is flukes.
Flukes are hard to tell, unless you have a severe infestation. That's why tattered fins are usually the first sign a fish keeper would find.
I think they are more common in laterally compressed fish and not often found on clowns?
Did you receive the clown that way?