"he's mediocre" is correct and usable in written English.
You can use it when you want to express your opinion that someone is average or not very good at something. For example, "Although John is a talented basketball player, he's mediocre at football."
As for the White House's feeble defence of her qualifications, it brings to mind the infamous justification for Richard Nixon's nominee, Harold Carswell: "Even if he is mediocre, there are a lot of mediocre judges and people and lawyers.
A week or so ago, Sen. Roman L. Hruska, of Nebraska, felt compelled to answer the charges of mediocrity brought against Judge G. Harrold Carswell, President Nixon's nominee to the Supreme Court of the U.S., by remarking, "Even if he were mediocre, there are a lot of mediocre judges and people and lawyers, & they are entitled to a little representation, aren't they?
San Diego quarterback Doug Flutie is 21-4 in home starts, and he was mediocre last weekend at Oakland.
Senator Roman L. Hruska of Nebraska praised Mr. Carswell, saying: "Even if he were mediocre, there are a lot of mediocre judges and people and lawyers.
Because Oshima's filming techniques were not wholly different from those of Godard or Rivette, he initially received a lukewarm reception: he was "mediocre" and had "nothing new to tell us".
And trying to defend a candidate who does not bear the imprimatur of an Ivy League diploma can be a losing battle, leading to comments like the famous defense of G. Harrold Carswell (Walter F. George School of Law of Mercer University) by Senator Roman L. Hruska: "Even if he is mediocre, there are a lot of mediocre judges and people and lawyers.
As former Senator Roman L. Hruska put it, defending a Supreme Court nominee from charges that he was mediocre: there are a lot of mediocre people, and "they are entitled to a little representation, aren't they?" As for me, I'd prefer to have somebody in office able to do the job well.
He didn't know he was mediocre, not genetically up to snuff.
I'd say he was mediocre but that would be a compliment.
After being brought back to the farmhouse, Bryce is told to accept that he is mediocre and ordinary.
