General Antonyms:
* Unimpressive
* Mediocre
* Ordinary
* Commonplace
* Unremarkable
* Dull
* Boring
More specific antonyms:
* Insignificant: If you're talking about something lacking in importance.
* Weak: If you're talking about something lacking in strength or power.
* Uninspiring: If you're talking about something lacking in motivation or excitement.
* Ugly: If you're talking about something lacking in aesthetic appeal.
The best antonym for "impressive" will depend on the specific context of your sentence.