In September of 2015, I computed the win ratio for Euchre players who had completed at least 100 games. I used the numbers that produce the "Absolute win/loss ratio" shown under your "stats" link. Below you will see this histogram. There's one player who has managed to attain a win ratio of > 70%! Only 15 between 60-70%... most players come in around 40-50%.
There is some speculation about just how good you can get with your statistics; how much of Euchre is a matter of luck, and how much is skill. Some people think the best you can do is somewhere around 66%... and these statistics support that idea.