Mid is usually a term to describe something unsatisfactory. Weather that be the look of a bird on a night out, or the quality of chips at the local.
"Oh god the reply was mid, cheers Taya"
Something average, nothing special
Waiter: How did you like the food?
Gordon Ramsay: Eh, it was pretty mid.
It's another insult you say to something or someone but on a lower scale like saying some thing is stupid.
"Bro that looks so Mid to use"
"Bro that show is so Mid."
Lol JoJo Mid Adventures