Have you ever heard the saying that "one man's food is another man's poison"? I believe this saying can be likened to other aspects of life, art included. Now, what do you think art is? To start with, art is diverse and is not limited to painting alone; it is a form of expression, of how we feel, and also a way we pass this feeling. After this, creativity is expressed in whatever form; it is left for the viewer to either understand and appreciate what the artist feels and tries to express, or not, but in the case where you don't just get it, that doesn't justify you calling the art bad.
What one person considers bad art can hold a deep meaning and be appreciated by another who relates to its meaning. The only form of art that should be considered bad is when it is generally not approved by the world's standard of either ethics, spirituality, etc, note the generally stated, and not it being to a specific group of people or religion.
More importantly, rather than spend time trying to subject an art to either good or bad, why not take that time to try to understand the artist's point of view, and its impact, try to understand what the artist was trying to express or communicate? How does the art make you and others feel? Does it engage or challenge you as a viewer? What thoughts or experiences does it evoke? I believe focusing on trying to analyze the art this way will shift your perspective on trying to cage art between good and bad.
This reminds me of a Nigerian upcoming lady artist, who was loved for her unique art, of making female portraits with realistic hairstyles, and over time gained many followers. As expected, there was a challenge on her to create more beautiful piece, to keep her viewers engaged and also continue growing her account, she decided to go out of her usual way to create a scary Halloween/zombie nun, now this very art received many critic, and many of her followers considered it bad, because she didn't create what they were used to, as most of these followers were religion bounded, and said she was drifting towards the satanic path, or has sold her soul therefore concluding the art is bad, how laughable.
But on a neutral point of view, the art wasn't bad, people gained a different meaning probably different from what the real artist was trying to express, even if it is the same as what she originally felt while creating, everyone is entitle to freely express themselves, even though it doesn't tally with how you feel, it doesn't give you the right to judge and consider it bad.
This same artist has been praised in previous art, but because she decided to express a darker side, probably to give way to more space for light in her, because that was what I felt the art meant.
In conclusion, the question of whether there's such a thing as bad art is a complex and not easily subjective one; the fact that an art doesn't meet your criteria or standard doesn't make it bad.