Focusing on quality:
* Clobber: This implies a work of art that is exceptionally bad, almost laughably so.
* Botch: This suggests a work that is poorly executed and full of flaws.
* Disaster: This emphasizes the utter failure of the work.
* Flop: This refers to something that receives a negative reception and fails to achieve its intended purpose.
* Junk: This implies a work that is worthless and lacking any artistic merit.
Focusing on impact:
* Failure: This highlights the lack of success in achieving the intended goal.
* Fiasco: This signifies a complete and utter failure.
* Flop: This suggests a work that is widely disliked and considered unsuccessful.
* Misfire: This implies that the work didn't have the desired effect or impact.
Focusing on intention:
* Amateurish: This suggests a work that lacks the skill and sophistication of a masterpiece.
* Uninspired: This highlights the lack of originality and creativity.
* Boring: This refers to a work that is dull and lacking in interest.
* Mediocre: This implies a work that is just average, neither good nor bad.
The best opposite for "masterpiece" will depend on the specific context and the aspect of the word you want to emphasize.