"Tu madre" at least in Mexico, usually implies a very offensive insult of course.
Although I think sarcasm is universal, its usage about mothers in law depends on individual sensibility and on how the "suegra" fits into the stereotype that they are very meddlesome, rude, gossipy, etc.
Still, sarcasm (for all kinds of topics) can often be charged with a lot more virulence than plain insults.
