Hi italo,
"lei" can be used as "você", but in a formal speech, so you are right, since the "cumpliment" she gave is not very polite, I guess she doesn't respect the guy too much.
The italian translation doesn't work actually, could it be "tu sei solo uno scemo"? How would you translate from brasilian portuguese to english Lilian?
Ahah now I get it, I have a lot of nice italian equivalents
"Sei un perfetto deficiente"
"Sei solo un cretino"
"Sei completamente idiota"
I could write worst, but this is not the place to do that and I think the meaning is clear