The latter - “let” is a diminutive suffix used to show that something is smaller than usual, in this case the brain.

Never sat well with me as I’ve worked with people with learning disabilities in the past. The point of the meme is to say “you’re so stupid/ignorant you basically have/you’re no better than those with a learning disability” but in my experience, people with such disabilities aren’t stupid they just take longer to grasp things (yes, I know it’s a spectrum). One of my friends at Uni had a visible learning disability and graduated first class for fucks sake and they looked and talked like you might expect somebody with such a disability would but you’d actually need to talk with them to challenge your own preconceptions and ignorance on the matter.

Anyway, my point is if you’re going to mock somebody’s ideas there’s no need to take a swing at an unrelated group of people. Disabled people of all kind already have enough of an image problem and the least we can do is not actively contribute to that.

And just to be clear, calling somebody “stupid” is not ableist in of itself. It’s that and reductively linking it to a disability which is the issue.