from a meaning of "average" to a meaning of "terrible"
this happens a lot actually, pejorativization and the reverse is a thing, hyperbole and irony means that colloquial terms for "good" and "bad" are always shifting. Think like, calling things "sick" as a complement
This is genuinely a completely normal linguistic process that happens all the time
I was always under the impression that "mid" was short for "middling," and that word had had a negative connotation for as long as I can remember. Akin to "mediocre."
