But Vimfuego you could interchange a positive word for "naughty", say "helpful" (a nice, patronising one I use on my DC ) and then extrapolate that your child thinks they are always helpful.
You say "you are helpful" meaning "you have just done something helpful".
The child hears "you are helpful" and thinks "my mummy thinks I'm always helpful".
So just say "that was a helpful thing to do". No confusion.
My 6yo is too clever to think that or to think that he is always naughty, even when I tell him that he is. My 2yo is a bit of an egomaniac (goes with the territory of being 2 IME) and probably thinks she is always helpful, but then she will argue that black is white, but hopefully the age of reason will eventually arrive.
I think that if I had only always said to DS "you are naughty" then he might be coming away with low self-esteem and thinking he is always naughty. But because I have said to him that he is naughty, helpful, clever, good, kind, generous, mean etc hopefully he realises that he is a normal human being with both good and bad in him.