When people say "it's bad so it's not art", they're challenging artistic value, essentially the same as what any art critic does. A cultured art critic who says "this is kitsch crap" and an unsophisticated man off the street who says "this crap isn't art" are both people expressing their dissatisfaction with the work. One expresses his thoughts in a way that is 'technically wrong' if taken literally and the other expresses his thoughts with jargon that is inaccessible to the general public.