I think that would be understandable, however, it is not perfectly correct.
I do not understand "mili", here... why is it "mili" and not another form of "nice". What case is this and do I assume one of the two people in this comment is a male?
You have four possible translations of "nice" here and you have to choose yourself, imagining with whom you are talking. So: Jesteś dla mnie miły (to John), Jesteś dla mnie miła (to Martha), Jesteście dla mnie mili (John and Adam, or John and Martha), Jesteście dla mnie miłe (Anna and Martha).
You could forget about "dla mnie" part and just say "You are nice" - this shows that it is simply Nominative.
Does the Polish sentence mean "In my opinion, you are nice" or "Your behavior to me is nice". I'm guessing it's the first but really, the English wording means the second.