Word for feeling amused by someone/people being ... stupid?

Is there a word for feeling amused by someone or some people do something ... erroneous or stupid ?

While not entirely restricted to your definition, I'd suggest schadenfreude,

pleasure derived by someone from another person's misfortune

In this case, you have defined the subset of misfortune which is produced by the subject's own actions, although the term also refers to misfortune inflicted without help from the subject.

ridicule which is contemptuous laughter.

mockery emphasises the disdain of such laughter.

(dated) ribbing, demotic from 'rib-tickling,' emphasises the social nature of mockery.

raillery, is exactly the meaning; but seems to me old-fashioned and formal.

RAILLERY Merriam-Webster

1 : good-natured ridicule : banter

However much M-W dresses it up, the banter seems to me derisive, scornful, or sardonic.

www.merriam-webster 'sardonic' disdainfully or skeptically humorous