General antonyms:
* Mediocre: Lacking distinction or quality; average.
* Ordinary: Commonplace; unremarkable.
* Unimpressive: Not making a strong impression; not inspiring awe.
* Poor: Of low quality; lacking in excellence.
* Terrible: Very bad; dreadful.
More specific antonyms, depending on the context:
* Ugly: (if referring to appearance)
* Unworthy: (if referring to merit or value)
* Miserable: (if referring to a state of being)
Example:
* Magnificent: The building was magnificent, with its towering columns and intricate carvings.
* Mediocre: The building was mediocre, with its plain facade and lack of detail.
It's important to consider the specific meaning of "magnificent" in a given situation to choose the most appropriate antonym.