I have no idea if this has any value at all , But today in game someone offered me a spawner for it because he said there is only a 2% chance of getting one from fishing , In wich case it would be a rare , but I have no idea if this is true. But if it is msg below what you would pay for it

 

I believe they are fairly rare, though I'm not sure

 

I agree clownfish are rare, but they have no good uses, they are like the same thing as a raw fish.

Source:
Clownfish - Minecraft Wiki

 

You should have taken the spawner for it, I would sell mine for a diamond each, and I would sell for even less if I wasn't trying to make a fish collection.