I have never been in a position where I was showered with unwanted food - usually I get offered something once and that is that. I've been to people's houses where I wasn't offered so much as drink from the hose and I find that far ruder than an over abundance of food. I think it is ruder to offer than not to offer, but to literally push food on someone kind of baffles me. If it's a casual visit I would be uncomfortable with a host going to the trouble.
I don't know enough about your MIL/DH and family traditions, food relationships
, etc. to give good feedback, except to warn you that maybe you don't have to turn this into a big fight. You seem to feel she's doing this as a PA move of some sort?
Why does she not eat herself when she cooks? That you politely refuse and she pouts is perturbing, but...
is the food bad? Would you turn it down if you were hungry? Is it just the timing/amount?
Since it is notoriously difficult to get someone else to change their behavior, don't try and figure out how to get MIL to stop making food: decide how to deal with her persistence on cooking. Maybe only visit her at dinner? If she' s a good cook ask her to show you a recipe; if she's a bad cook you could offer her your favorite.
If she's old fashioned you could bond with her over that matriarchal "job" of feeding the boys - since that job has passed to you with your marriage to DH...maybe she's jealous he's "left the nest" and is psychologically shoving "worms" down his throat when he comes home?
Okay that last one was a total spitball and I'm blaming it on the pain meds and lack of sleep.
But back to your trouble...
If the problem is just too much food at the wrong time, I think you should have DH tell his mom either of the following depending on whether you (a) don't want the food the food or (b) want the food but not so much/right then:
A: "Mom, we appreciate your wanting to be a good hostess but we really came to see you, not to eat - how about coming out of the kitchen and talking to us?"
B: "Oh gee Mom we appreciate the effort but you shouldn't go to so much trouble every time we come here. I know you want us to enjoy your food and no stuff ourselves sick. Can you put this up so we can enjoy it together later? We came to see you of course, not to eat!"
You might change when you visit her so it isn't during "meal times," or tell her "Oh, we don't want to interrupt your cooking so we'll visit another time" Basically, try and communicate to her that you want to see her when you visit, not just eat her food. Maybe bring a game or something to distract her and keep her out of the kitchen? Just try and brainstorm ways to either detour her from cooking or keep from receiving so much of what she cooks.
Other brainstorms: Is your mom feeling neglected or needing to be useful? Maybe she could find a hobby centered around her cooking - competitions or doing it for charity, etc?