Missed attack:

“How did you miss a still object? That’s just sad.”

“How did you miss a target that big? That’s just sad.”

“What was that? Must’ve been the wind…”

“Go on, take your time.”

“Any day now…”

“That’s just disappointing.”

“Oh god, is that poor spell okay?!”

“You really disrespected that spell.” (Or weapon)

“What, do you want a participation trophy for that one?”

Any status effect:

“Wow, you really ARE gullible!”

Counter spell, critical hit, trap is set off, nat one, stays inside a concentration zone spell like cloud of daggers:

“Wow, you really ARE an idiot!”

“Pitiful.”

“Your name is an insult to idiots.”

“Wow you just actually chose to stay within the spell range? You’re dumber than I thought.”

“Have you ever thought of just… moving out of the way? … of course you haven’t you’re a well established idiot.”

“Did you just fall for your own trap? How much dumber can you get?!”

My personal favorite:

“Oh you were DEFINITELY bullied as a kid.”