It's somewhat mediocre because they're not strikers [boxers or kickboxers], they're MMA fighters. And it's only mediocre when you compare them to professional boxers/kick-boxers, which should be expected.
90% of MMA being terrible strikers is still a horrible exaggeration. For example, let's look at the top 15 LHWs [in no particular order]:
Jon Jones
Machida
Dan Henderson
Phil Davis
Lil Nog
Rashad
Gustafsson
Glover
Thiago Silva
Shogun
Bader
Mousasi
Te-Huna
Feijao
Hamill
Which one of these fighters has 'terrible striking'? According to your 90%, a good portion of these fighters should have terrible striking. Since it's top 15, it won't obviously be 90%, but it should still be a decent amount.