Yeah, this is what I meant when I said the script might not want to lampshade something that is fundamentally silly. Den-O would've been completely unbearable if they'd had other characters always react to the changes of hair and costume that the Imagin caused realistically. Lampshading the impossibility of the quick transformations would've gotten annoying very quickly.
A lot of things people would realistically do in a given situation are also repetitive and, frankly, kind of boring. Den-O would not be a better show if Ryotaro's friends had constantly gone "Hey! His hair and voice and clothes changed again, that's weird!" or passers-by went "Oh my God, what happened to that guy!" all the time.
Instead the script just left the changes of hair and costume to be what they were-- visual flourishes used to make it very clear to the audience, at a glance, when Ryotaro was possessed and by whom. Focusing on the transformation's mechanic any more than that would've just slowed down the action to no real benefit.
So I suspect we didn't see a big scene where Hina was all "Brother! What happened to you, why do you look so strange?" because honestly-- is that interesting? Yes, it's what most people would do, but what is the point of depicting it if it's not intrinsically interesting or entertaining or relevant? There isn't one.