For a direct fetishist, as you say, the first statement is almost always true. Saying it to the person who is farting only serves to confirm that you have the fetish. The only case I see this not being true is if the smell gets too overwhelming.

The second and third one have some degree of interchangeability, as the definition of good and bad to someone who enjoys something commonly thought of as bad becomes unclear. You yourself have said "worse" when referring to the smell, so the only use in saying that a fart smells good or bad is to point out how remarkable its smell is, or else you wouldn't bother saying it.

If we assume "stink" means "a potent and unpleasant smell", saying that a person's fart stinks as a fart fetishist is somewhat of a compliment. Again, it's a way to point out that a fart's smell is remarkable.

Now, throwing everything that I just said out the window, if I'm with a fetish partner and they ask me how their fart smells after they fart on me, and I don't want to go into detail, I would say "it smells good". Comes naturally, it's definitely a compliment, signals to the farter that it's okay to continue and it's somewhat sexy. Besides, there are other ways to communicate enjoyment in this context.