I don't think there really is a way to measure "skill level". Number of kills in a tour obviously does not show skill. The only thing that comes close to "skill level" Is K/D ratio. Even that doesn't really show skill.
Someone could have a K/D of 10 easily if they only fly 262's and other l33t planez, while a P-38 pilot with a K/D of 5 could be a much better pilot.
Besides, this falls under the category of "gamey".
Although I wouldn't mind seeing little X's on my plane for every 5 or 10 kills, reset each tour. I can imagine that would cause some technical problems though.