this isn't even a close call for me
if my mother trashed my wife when we were together, or even now actually, i'd shut her down without a second thought.
your husband's mom's job is to support her son's marriage.
if she feels she can't in good conscience do that then it's her job to get the hell out of the way and let her son live his life.
your husband has to realize his mom, when trashing you, is dissing him as well, telling him through more than strong inference that he's made very bad choices when he married you.
the more pertinent question you need to ask, directed at your husband, is, 'do you agree with your mom'
and the next question you need to ask, directed at yourself, is 'what's wrong with my marriage that i don't clearly understand how my husband feels about me'
andand sure you could let the mominlaw have it with both verbal barrels, but in the end what will that accomplish except to slide you into the mud with her, which very well may be where she wants you to be.
it's your husband's job to make this right, and if he doesn't then you need to take a hard look at the 'man' you married.
andandand ignore the granny. she's just having a very senior moment most likely.
as a matter of fact you could have some fun with her
call her back and thank her for the kind words
that would compliment her confusion nicely.