General antonyms:
* Mediocre: Lacking distinction or quality.
* Ordinary: Commonplace, unexceptional.
* Dull: Lacking interest or excitement.
* Poor: Of low quality or standard.
* Unremarkable: Not worthy of notice or attention.
More specific antonyms:
* Ugly: The opposite of beautiful or aesthetically pleasing.
* Cramped: The opposite of spacious or grand.
* Miserable: The opposite of joyful or splendid.
* Cheap: The opposite of expensive or luxurious.
* Simple: The opposite of elaborate or complex.
The best antonym will depend on the specific meaning of "magnificent" in the sentence.