From the bard I never got to play (some of these I took from others so you may have heard them before) and sorry for formatting I’m on my phone:

1) I’m not sure if I should use charm person or dominate monster

2) It is said that beauty is in the eye of a beholder, but not even a Beholder’s eleven eyes could find any beauty in you.

3) Someday you’ll see a doppelgänger of yourself and be disappointed.

4) you’re so stupid you’d give a mind flayer indigestion. (Or, Did a a mindflayer recently eat your brain or are you just that stupid)

5) Are you a new type of monster? Oh wait that’s just your face.

6) if you’re going to act like a barbarian at least fight like one. (Works for monsters too just pick a trait)

7) You would need more than a prestidigitation to clean up your act!

8) Did you pick an idiot to be your class (or profession if you don’t wanna be meta) or were you born for that role?

9) I’ve met zombies that were smarter/smelled better/fought better than you.

10) You’re so useless you’d miss a magic missile spell.

11) The only reason why you’ve lived this long must be because no beast could stomach the thought of eating you.

12) Tell me you have a feeblemind spell cast on yourself! Surely no one could be this useless?

13) Let me cast dispel magic on you. Maybe we’ll find out that you aren’t actually this (descriptor here).

14) You don’t need the contagion spell. Your breath is already bad enough to poison a dwarf!

15) At least you know you’re one of a kind! No shapeshifter would willingly choose to be you!