I think that's a bit harsh on Mendy, as Karius was pretty clearly a below-average keeper who could never hope to put up a season as good as Mendy's was last year. Mendy was the 9th best shot stopper in all top 5 leagues last year and his team qualified for the CL, so it's not like they were being dominated week in week out which can distort goalkeeper stats.
I also don't think Mendy is particularly error-prone - off the top of my head this season he's made only 2 errors leading to goals, those being the hesitation in sweeping up vs Leeds leading to the Bamford goal and the sweeping error conceding a penalty vs Everton. He also kicked it straight to Lacazette but recovered and I'm sure he has a couple other giveaways, but I don't think he's making mistakes more often than other top keepers.
For sure you're correct that he's benefitting from the comparison to his predecessor, but I just would hesitate to say that he's not good enough to play at a top level especially given the endorsements from Cech and Lollichon. I could end up being wrong but again I'd reiterate that Chelsea have far bigger problems than their goalkeeper.