"mediocre children of alumni" This is an unfair description of the advantage of legacy at an elite school. It's typically an edge, like playing a sport or belonging to an under-represented demographic.
And if the impact is large - equivalent to 1 point of GPA - then there are legacy admissions a full GPA point worse than the least capable of their non-legacy peers, which is even worse.
I agree mediocre is inaccurate, because mediocre means half way to the top, and students who need legacy admissions are at the very bottom of the class.