Mayor to House usually is a promotion yes, but there's a line where it's not anymore. deBlasio won't run for congress, Lightfoot won't, Breed probably not either, etc.
cities over I'd say... 500k are where it gets fuzzy. Mayors of cities that size govern over as many people as Gov. Mark Gordon. In congress most reps are just unheard unseen nobodies. Mayors on the other hand, with a large enough city you are seen far more often.
Of course this is mainly talking about non-figurehead type mayors. Wayne Messam for example would unequivocally be promoted if he ran for house and won because Miramar has a pretty powerless mayor from my understanding.
Isn’t Greg Stanton being elected to AZ-09 a promotion because Phoenix has a weak mayor system?
That’s what I’m talking about at the end. However for a stronger mayor it would be a demotion