This. Cosplaying them in their official looks is ok because the outfits, hair and make up are pretty much a costume for them as well, their "stage persona" or even a "character" they play. Plus besides looking awesome, replicating those looks can be as hard as replicating an anime character's ones.
Is no different from people dressing as Gene Simmons's Demon.
Now, a "casual x cosplay"? that shit is not cosplay. It doesn't work, like a "casual Gene Simmons" cosplay wouldn't work.
That's when things start getting creepy as fuck, and into roleplaying territory.
How can it be a cosplay if you are dressing in casual clothes? There is nothing special or unique that makes the conection to the guy you are "cosplaying as", you are someone in casual clothes and that's what people will see, not your favorite artist.
Do you like their casual looks so much you want to dress like that? Even get the same hairstyle? Do it, but don't take selfies and claim is a cosplay. Is not.