Neutral:
* Unfunny: This is the most straightforward and neutral term.
* Humorless: This implies a lack of appreciation or understanding of humor.
* Unoriginal: This describes someone whose jokes are stale or overused.
* Clichéd: Similar to unoriginal, this refers to jokes that rely on tired tropes.
More critical:
* Trying too hard: This suggests the person is putting in extra effort to be funny, but it's not working.
* Unintentionally hilarious: This is ironic – the person thinks they're being funny, but their attempt fails so badly that it's actually amusing.
* Cringeworthy: This describes humor that makes others feel uncomfortable or embarrassed.
* Cheesy: This implies the humor is corny or overly sentimental.
Other options:
* A comedian in their own mind: This is a humorous way to say someone thinks they're funny but isn't.
* A failed comedian: This is a more harsh way to describe someone who isn't successful at humor.
The best term to use will depend on the specific situation and the tone you want to convey.