She wanted to say something nice about him, but it may be that she would have been better off saying nothing at all. I looked up "Pareto optimal" and went to Wikipedia.
Pareto efficiency or Pareto optimality is a state of allocation of resources from which it is impossible to reallocate so as to make any one individual or preference criterion better off without making at least one individual or preference criterion worse off.
So what she is saying is that he is the best he can be, considering what he has to work with. Not exactly high praise. She may as well have said, "Your feet don't smell too bad when you take a bath," except that he would have understood that easily. This comic is from Zach Weinersmith at Saturday Morning Breakfast Cereal.