Simpleton is usually used to insult someone's intelligence, so be careful when using it. Calling someone a simpleton in an argument can be quite offensive.
Pro Tip 2/3
Not Literal
Sometimes, 'simpleton' is used hyperbolically to exaggerate someone's mistake or gullibility. Don't be such a simpleton; everyone knows not to trust emails from unknown sources.
Pro Tip 3/3
Colloquial Use
'Simpleton' is conversational and might not be suitable in formal writing or speech. During casual talk, he jokingly called himself a simpleton for forgetting his keys.