Mrs. Joe, or more commonly 'so Mrs. Joe', is a phrase used to describe a person who is being bitter, salty, and just a nasty little meaniepants. It can also refer to someone who is hateful almost to the point of verbal and emotional abuse, although it is more rarely used in that context.
'So Mrs. Joe' is primarily used by cool and intelligent, but perhaps slightly socially inept, teenagers who find great humor in injecting tidbits from their high school assigned reading list into day-to-day conversation. Based on the character Mrs. Joe Gargery from 'Great Expectations'
Mechlech: your lack of appreciation for Kafka horrifies me and makes me question your place in our friend group
Herford: what friend group? we're just two people
Boris: how darest thou?! note mine presence, toad!
Mechlech: Shakespeare is so mainstream. and your grammar is flawed. stop pretending to be an intellect, thou sodden-witted lord
Herford: yeah, Boris, you need to do something to prove that you're not a poser
Mechlech: (coughs) says the person who refuses to recognize the importance of eastern European literary greats
Boris and Herford: Mechlech, you are SO Mrs. Joe