Was listening to Arthur C. Clark's Rendezvous with Rama and seem to have discovered the origin of simp. It's not from "simpleton" as wikipedia says, but rather "superchimp".
Some excerpts, ch. 11 p. 40-42:
"There were four superchimps aboard Endeavour ... Unlike the monkeys who were their nearest relatives Endeavour's simps were docile, obedient and uninquisitive. Being cloned, they were also sexless, which eliminated awkward behavioural problems. Carefully housetrained vegetarians, they were very clean and didn't smell; they would have made perfect pets
"the only rule regulating shipboard sex was 'So long as they don't do it in the corridors and frighten the simps'
"each could replace 2.75 men for housekeeping, elementary cooking, tool-carrying and dozens of other routine jobs. ... simps were quite happy to work fifteen hours a day and did not get bored by the most menial and repetitious tasks.
It's not exactly the meaning of today, but way closer than "simpleton", and this is from 1973 so predates all the other known uses except as idiot/fool.
Not much to discuss, but I thought you guys might be interested in this little bit of history.
I always liked the implication of simpering. Shameless self-demeaning subservience.
There's uses from 1900 on that you can substitute idiot/fool and those statements make sense, so they are shortened from simpleton.
I don't think you can make sense of most modern usage by replacing it with idiot or blockhead, but replacing with 'sub-human, servile, sexless pet' seems like it keeps most of the meaning.
People used to read books and this is a famous one. I'm going to say the science fiction community has a disproportionate amount of simps so this could easily be a witty insult directed at them by referencing their own material.