I'm weirdly balanced between applauding your attitude to donating whatever they want, and wondering at what you consider to be poor with your cake mix story!
I come from a working class family, but there were times we dropped below the poverty line. I grew up with utility shutoff notices coming in on a regular basis (and the last minute scramble to pay it off hours before the shut off would occur), bill collectors routinely calling (and we kids had to answer every phone call to screen them out and say our parents weren't home), my parents having to take the sky high interest rate at a shady used car lot because they didn't have decent credit or the cash available to buy something outright, buying clothing at thrift stores and yard sales (I don't have a problem with this, but back then it was a necessity for us, not a frugal and eco-friendly choice), etc. There was a period of time we were on AFDC, living in a dilapidated house converted into three apartments that were infested with rats and insects, literally stepping over drunks in the gutter on our way to the bus stop, where the neighborhood kids (including us) would run over to the Salvation Army and grab the out of date bread products being dropped off outside for the food bank before the volunteers had a chance to get it into the building. I've had a Christmas where the only gifts we got were the ones from the people the Salvation Army gave our names to, along with a cat my mom adopted for free on Christmas Eve in the hopes he would take care of the mice and rats. So I've been poor by just about every standard definition out there. And as bad as it was, I know many people had and have it worse.
A cake mix and can of frosting is cheap compared to almost any other way of getting a birthday cake, and if bought on sale and made with sale eggs and cheap vegetable oil, is cheaper than making a homemade cake. I'm not saying only poor people use cake mix -- I'm saying that for us, the cake made with a box mix and can of frosting made our birthdays special, and I'll donate it if the food bank wants me to.
One thing that nags me is how some people think the poor should be able to do certain things because it's never occurred to them how different it is when you truly don't have enough money. For example, so often the advice is to buy in bulk, but you have to be able to get a little bit ahead with the grocery money before you can do that. Or they should make a cake and icing from scratch, but you have to buy all the ingredients and if you aren't going to use them all (for example only need 3 cups of flour, half cup of cocoa, a teaspoon of vanilla, etc.), you've spent money on food you don't need.
I won't judge what the food bank asks for. Right now they want boxed stuffing, canned green beans, cream of mushroom soup, boxed mashed potatoes, and canned gravy. Those aren't the foods I eat at Thanksgiving, but I'm not above buying them for people who will eat them. The ability to judge others for their food choices speaks of significant privilege.