Focusing on quality:
* Clobber: This implies a poorly made piece of work.
* Botch: Similar to "clobber," suggesting a failure.
* Dud: This implies a complete lack of success or impact.
* Trash: A very harsh term, implying something completely worthless.
* Failure: This is a general term for something that didn't meet expectations.
Focusing on skill or artistry:
* Amateurish: Lacking skill and professionalism.
* Mediocre: Average or unremarkable.
* Clumsy: Lacking grace and skill in execution.
* Crude: Roughly made or lacking refinement.
Focusing on impact or effect:
* Failure: This emphasizes the lack of positive effect.
* Flop: A complete lack of success or impact.
* Disappointment: This focuses on the negative reaction it elicits.
The best antonym depends on the specific context of the word "masterpiece" and the intended meaning you want to convey.