I think it is fine - I often tweak monsters to fit the level of the party.
I don't think it is and I would despise my DM for condescendingly giving me a fight with a dragon at level 4 (or 6) just to make me feel good about myself, but then again giving the players a fight is not exactly the point of the thread.
The issue is the consistency of the presentation. Every time you have an adult red dragon in the game, it should be an adult red dragon, period. It shouldn't come in a dozen of scaled variants to fit whatever level the party is at a given moment.
And yes, crippling a monster to make it less challenging than it should be on the contrary can be a legitimate idea for an encounter, as long as the game acknowledges the difference explicitly and I don't have to make up a shitty fanfiction in my head to justify it.
