Listen, if you're tuning into a drama called "The Heavenly Idol" with even the slightest awareness of its premise: manage your expectations.
Everything about this drama was not overly ambitious -- a shaky budget drama with a repetitive episode formula, a fun spark of chemistry, and decently charming actors. I believe that everyone involved HAD to know that they were making a show that people could throw on after a long day of school or work and just not think too much about. It didn't make a whole lot of sense, but it was a novel idea and it tied things up pretty tightly in its ending.
While every single aspect of it could have been far better (music, portrayal of idols, fantasy lore, length, production value) its a fun drama to pass the time with.