The suffix "-off" is often used to generate a term that means a contest or competition, with the stem describing the quality or activity that the participants are being compared on. For instance a "cook-off" or "dance-off".
In this case, "mid" means in the middle (as in "mid-September"), and has a slang use to mean "mediocre".
The tweet is sarcastically saying that neither of the women is particularly exceptional, they're figuratively competing to see who can be more mediocre. The contained tweet seems to be one of the girls ("me" on the left) claiming to be better than the other ("you" on the right).