An insult focused on hurting someone that is deflected easily or has lost it’s meaning because either that person doesn’t care, is what you are describing, or the roast/insult just makes no sense and sucks.
Here are some examples:
Moron: Hahaha you are a nerd. Probably Asian
Smart kid: I am Asian?
Moron: Oh ok
Smart kid: That was unsulting
Moron: Whats that me-
Smart kid: FUCKING GOOGLE IT MORON
The unsult: virtually anything ever said by characters Michael Scott or Dwight Shrewt of The Office. Pam or Jim, intervene to reveal the full ineptitude of the unsult.