Aspects created via Declaration seem to me to be just as valid as other Aspects that are "created" temporarily on established PCs through game mechanics.
Just as valid, yes. Just as temporary, no. Declarations are assumed to be permanent facts, have always been facts, but have only just now come into focus.
YS 116:
"As with assessments, aspects created with declarations don’t go away after being tagged, so long as circumstances make it reasonable that they hang around. This does mean that occasionally assessments and declarations can backfire on the character establishing them (other characters might use the same aspect, or the GM might bring that aspect back around to complicate the character’s endeavors)."
But that's not the point - the point is: should PCs get Declarations made about them. I think not, but your game may thrive with the option. Especially if your GM doesn't limit characters to only 7 Aspects.
Edit: But oh, heaven help you if your GM's NPCs are fond of making Declarations like "Wall-Eyed," "Skips When Running," "Shoots like a Stormtrooper," or "Broke-Broke."