Is there any resource to research dumb people way of talking without knowing a real life dumb people ? When I do a dumbing down scene, I feel like I am just copying others TF dialogue, never met someone that has a “dumb accent”. How do you like to show dumbness in your characters ?

There’s no specific way that dumb people talk, really. Short words and simple sentences are the best way to go, if you want a neutral “dumb” sort of speech. If you want to go dumber, I suppose–or if you’re trying to highlight how far a smarter person has fallen, having them misuse large words they may have known before, or have them get lost in longer sentences can be a good way to do it. More than that? Start fucking with their grammar. subject verb agreement, shifting tense, wrong verb forms–those sorts of things, when used sparingly, can help as well.