The accepted answer count is already taken into account in your reputation (there's a bonus equivalent to 1½ upvote for each accepted answer). I don't think it's a good idea to cutter up the answer signature with yet another number.
I also disagree with your formulation: an answer can be accepted even if incorrect, and (more commonly) can be correct but not accepted. (Are you saying that Pekka has a 20% incorrect answer rate?) This number should really be called “accepted answer rate”, or even “accept rate”¹.
It would make sense to show this statistic somewhere in the user profile.
There could be a tab in the
/users page too, with users (who have more than N answers in the time period shown) sorted by accepted answer rate. In the meantime, you can look up this number on the data explorer, e.g. Top users by accept answer rate.