Perhaps you are correct. I don't really know what the historical record is.
I would say that if the country voted about 50/50 in the election, then I'd think that 50's approval is about right.
As for Clinton at that time, he was able to contribute to the first Republican Congress being elected in 50 years.
Mediocre means..."neither good nor bad, average, ordinary" and I think that is what Bush has been, 9/11 and associated events excepted.