This filter actually measures statistical divergence from standard English rather than stupidity but the link is definitely worth a visit for the eminently t-shirtable FAQ entry:

Do you really expect to be able to detect and filter anything that's conceivably stupid?
No, of course not. You'd need real AI for that, and beyond a certain point it's simply subjective; after all, a sufficiently advanced AI would probably filter out the whole of human discourse, which isn't the idea.