The phrase "mediocre acting" means that someone's performance or acting skills are average or not very good, lacking the excellence expected or desired. Full definition
Most kids with mediocre ACTs (the average Colorado score is 20.1) would be lost at Harvard, and would be less likely to write a persuasive recommendation for a senator, too. (apluscolorado.org)
But the bad story and mediocre acting from Teresa Palmer and Sullivan Stapleton, make Kill Me Three Times a movie strictly for Pegg fans at it's finest. (metacritic.com)
SUMMARY OF THOUGHTS: Despite having a couple of cool moments, I, Frankenstein is mostly just a mess of a movie — with mediocre acting, terrible CGI, and poor writing. (weliveentertainment.com)