Their lyrics are filled with brutality, sexuality, obscenity, and especially in their earlier works, slurs. The songs describe taboo topics, turn everything Up to 11 and exceed all kinds of tolerable limits. "Panty Shot", "Faggot" and "Backmask!" are popular examples for this. However, they know exactly what they're doing, and none of it is supposed to be taken seriously, since it's so highly exaggerated.
In the booklet of Frankenstein Girls:
You must listen to this album with your tongue planted firmly in your cheek. If you don't have a sense of irony, or you are a piece of shit bigot, or a fundamentalist anything, return this album immediately, don't come to our shows, don't wear our t-shirts. Fuck off. If you're a parent who neglects, beats, or doesn't show their child love, know that we don't condone violence so don't waste our time in court while you try to explain to the jury how our music made our "happy & normal child" into a deranged lunatic.
Vulgarity was the name of the game for their earlier shows. Jimmy was known for waving his privates about and once tried to set them on fire. There is footage of him bringing a crowd member on stage before making out with and then teabagging him. Stuffed animals would get thrown on stage, and Steve and Jimmy would pretend to fuck them. One specific case: Jimmy drank his own urine before passing it to Steve, who took one sip and spat a second into the crowd. Jimmy then began making out with audience members.
Growing the Beard: In terms of commercial success, definitely. In terms of critical success, that's a topic of debate right there.
Hilarious in Hindsight: "Bullshit" features the line "it's no accident, you all pay my rent, and I am free to download all the porno from the internet." With the advent of Patreon, a monthly paid subscription service, an artist can pretty much have their audience do just that.
Yoko Oh No: Lyn-Z's marriage to Gerard Way. It should be noted that this applies to the rabid sections of both Mindless Self Indulgence and Gerard's band My Chemical Romance. That said, both bands actually get along pretty well.