I grew up with dumdums and tootsie rolls doing this because of how the machines made batches of different flavors. The mystery flavors were just mysteries because during the change between flavor productions, there's a margin of error so they just wrapped the candy made in between two flavors as "mystery flavors" since they couldn't be 100% sure what flavor it was gonna have anyway.
I don't think that's what Mountain Dew is doing though. They totally know what flavor it is, they just don't wanna tell you it's dog vomit and hairball.
I enjoy trying limited time flavors (of various brands and product types) and mystery flavors are no exception. My wife likes to as well so we'll usually buy a single bottle to share and try. Despite liking the novelty it is pretty rare for me to leave the experience thinking, "I need to clear out the shelves before this goes away."
Most of the time, yes! I will avoid anything that’s clear because it makes my brain not want it. Unless, I’ve looked up the mystery flavor (in the case of a static mystery flavor).