As an english person i can pretty much confirm "stupid" isnt a swear word, theres a huge difference between swearing and using language which may be slightly undermining. Stupid in english can be anything, we can call songs stupid, people stupid, hair stupid.. It's not insulting. Too many people are getting insulted by normal standard words used in every day language which aren't swearing at all. I call my friends stupid all the time joking around with them, its pretty much the same as "silly"..