I think we should distinguish between different terms: legal, ethical, and sporting.

Flossing is illegal in Washington. So that answers one question.

Some would say flossing is not sporting since the fish is not willingly biting. But it still takes some amount of skill to drift a line into a fish's mouth, I'd say. Even moreso to do it without snagging the fish in some other body part. Is hunting "not sporting" because the prey is not willingly walking into the bullet/shot/arrowhead? I'm not sure why the willing take is the only way for fishing to be considered "sporting" (even though that's the part I like the most).

As for ethical, I'd say flossing differs from typical snagging if it's intentionally done in a manner that allows for safe catch and release, which is not the case with typical snagging (i.e., ripping the treble through the water trying to catch any part of the fish).