There's a subtle but important difference between people of lesser cognitive ability and people who choose to believe dumb things. I think mostly everyone on this website would agree that it is bad to make fun of people in the first category. People don't choose their cognitive abilities, they are more or less born that way and deserve to be treated with respect regardless. However, people voluntarily choose to believe stupid shit all the time, regardless of intelligence, usually to feel superior to other people (e.g. conspiracy theorists, racists, etc.). It is important to note that the latter group makes a CHOICE to act stupid, and thus deserves ridicule. And since brainworms aren't a real disease that real people suffer from, I think it is generally okay to use as an insult. On the other hand, using this term to refer to a person with cognitive impairments would be ableist IMO. Then again, so would calling them "stupid". I think it really depends on context.