I think that the terms "bad" and "good" are problematic when describing artistic endeavors, whether commercial or not. Art is subjective, with subjective judgments, and therefore, "bad" and "good" aren't clear enough to communicate the quality of a piece of art, from one person to another.
(This is the part where it starts to turn into math.) I think it would be more complete, more honest, and more coherent to judge art on (at least) two dimensions: Accessibility and Conventionality. I'd like you to imagine an X-Y graph. The X-axis (horizontal) goes from Inaccessible to Accessible:
Having "Inaccessible" on the left doesn't mean that it is a bad thing; having "Accessible" on the right doesn't mean that it is a good thing. This continuum is meant to compare a piece of art with its potential/actual audience. There are actually several dimensions to this one-dimensional line. Art made for niche audiences could be considered less accessible than art made for wider audiences. Art that is commercially successful, whether intended to be niche or not, could be considered more accessible than a formulaic commercial failure. A piece of art, like a specific song, might be accessible to a teenage audience, but inaccessible to an elderly audience, and vice versa. A melody using the Do-Re-Mi major scale, or its modes, will probably be more accessible to Western audiences than a melody using a more complex, perhaps "exotic" scale. A Do-Re-Mi melody played out of tune will be less accessible than a Do-Re-Mi melody played in tune. Rating accessibility is to compare art with audience.
The Y-axis (vertical) goes from Conventional to Unconventional:
Now we have a two-dimensional graph to evaluate, judge, and critique artistic things:
I must confess that my own personal biases have placed "Accessible" on the positive side of the X-axis, "Unconventional" on the positive side of the Y-axis, and arbitrarily putting Accessibility on the horizontal and Conventionality on the vertical. Personally, I am more inclined to enjoy books/TV/film/music/painting/sculpture/etc. that is relatively easy to understand (Accessible) but is not a repeat of previous work from others (Unconventional). Most popular media tends to be accessible and conventional, any way you cut it. Most amateurish attempts at art tend to be inaccessible (poor form, poor technique, etc.) and conventional (attempting to be trendy and popular). On the other hand, potentially brilliant, underrated art tends to be inaccessible (does not pander to an audience, or does not want an audience) and unconventional (does not adhere to genre or zeitgeist).
Perhaps we'll need a third continuum (Z-axis) to differentiate the trial-and-error beginners from the misunderstood geniuses ... but is there really a difference, from the end-user's (audience) point of view?