I think this could be based on some weird Freudian association with her last name, which also starts with the same letter.
She makes a few exceptions, of course, including pork and peanut butter. But everyone likes those, so they don't really count.
So with that said, Mary has not endorsed this pesto; it is for the penniless, not the picky.
The reason I can say it's cheap is that it's made with cheap grocery store green tube parmesan, swiss chard (the whole bunch was $1.50; I used literally 1/10 of it), a little Kroger-brand olive oil –– you could substitute canola if you wanted to save even more money –– and cashews.
To make it, I threw all these ingredients in a blender, starting off with the oil:
1/4 cup olive oil
1 cup torn swiss chard
1/4 cup cheapo parmesan
1/6 cup cashews
Then I blended it all until it turned into magical velvety green mush.
Because the cashews were salted, I didn't add any seasonings. Garlic and black pepper probably would have been tasty, but this was fine on its own. The entire process took less than 10 minutes.