The fish doesn’t really look like it has dropsy to me.
It kind of looks like the fish dislocated its jaw - usually the only remedy there is to pop it back into place (gently pushing in) - easier said than done on a piranha…
Does it still have a feeding response or no interest at all?
It could be a columnaris infection - that can be around the mouth. I’ve had luck treating it with seachm polyguard.