The horizontal tails on the F-35 resemble the original tails on the
YF-22. I assume that the F-22 tails were changed to make them
stealthier, which would imply that the F-35's tails are unstealthy. I
know that the F-35 is supposed to save money by concentrating on being
stealthy from the front, but I don't see why it would be expensive to
reshape the tails in the manner of the F-22. I wonder if the F-35
exhaust is so unstealthy that changing the tails would be pointless.

Are the F-35 tails a detriment to stealth and if so how much?

Hard to say precisely, but I have it on good authority that the airplane
isn't THAT stealthy. So tweaking the tail seems fruitless, eh?

R / John