@icantnotthink Well, there's an important thing at play here: Most manga here have gone through an editor first (weeding out the really rubbish-quality) and then through a scanlator second (ensuring they're at the least good enough for someone to bother scanlating them). As a result, it's very unlikely to find a manga here that's so bad it's unreadable.
(A way to escape both of those filtres would be to just make an English-language comic oneself and upload it here. TedXSarah do this, and, surprise of all surprises! They're ranked in the bottom 0.25% of all titles on MD.)
I would personally like to see a percentile rank for the manga on MangaDex. In fact, out of curiosity, I went and did a quick investigation, and my jaw dropped. Are you ready to hear the results?
A rating of 7.87 is decidedly average, because half of all manga that have at least one rating here have a score higher than 7.87. A rating of 8.45 means the manga is among the top 5%. A rating of 7.25 means the manga is among the bottom 5%.
95% of all manga on MangaDex (that have at least one rating) are within the 7.00 - 8.65 range. Here are some other ranges for you:
75%: 7.54 - 8.13
90%: 7.25 - 8.45
95%: 7.00 - 8.65
99%: 6.40 - 9.04
So, while the legend describes a 6.00 rating as "fine", in practice this means that the manga is in the bottom 0.25% of the entire site, ie page 250/309, at the very bottom of which start the 0.0/10, ie "no-one has even bothered to rate this yet".
So,
@EnRyuu_surf kind of has a point: The rating this manga has pins it as pretty mediocre, compared to most other manga here. I won't contest the rating (if you ask me, "Jikai Death-game ni gokitai kudasai!" is the far and away the most underrated manga on the site) but he is allowed to be miffed at this being judged as basically just par.
Oh, and
@GalladeGuy: Gal Cleaning is ranked second-worst, right behind the undisputed king of bad manga that is "Mina-sama no Omocha desu".