I'd like to add my 2 little pennies and agree with the masses-don't say anything. The way toddler eating has already been described here is fairly accurate-and unless the child is obviously malnourished (like, in the hospital dying from it), it's always best not to question another's parenting. I'm sure your mom means well, but she is wrong.
Just as an example, my oldest daughter was so picky that I thought she lived on air for the first couple of years of solid foods. Seriously, she wouldn't even drink chocolate milk for the first year off of formula. We had one major blow-out over macaroni and cheese when she was about three or four-it was spiral noodles instead of elbows and she absolutely *would not eat it*. I must have been having a long day because we really got into it-I was even yelling at her-but she still ended up winning that battle of wills. These days she eats a bigger variety of foods, first because she is 14 and is more easily reasoned with, and second because we instituted a "just have to try it" rule at the table once her sisters came along. Eventually she came to know and love foods that weren't pepperoni, but it took a while and she still prefers the spicy sausage.
As it turned out, she has mild autism, and the aversion to most foods are probably related to sensory difficulties. We used to get a lot of crap over not "making" the poor child eat, but it really wasn't worth the meltdowns. I eventually stopped listening when someone "said something". The concerned citizens in question didn't usually know what they were talking about.