Yeah, so let's talk about that bolded part for a minute.
The way I see FFXIV right now is that it's a game that has mostly two sides: an old school side (Basically anything pre-stormblood) and a modern style (anything post shb). So, the old school style is what I find mechanically engaging and it definitely appeals to a type of player. You had to pick up some skills from other jobs to give you boost in the one you were playing (raging strikes for dps, swiftcast for casters, flash from pld to help keep aggro with war, etc). Aggro was everyone's responsability, even if some people didn't want to bother with that. Repose actually had some use in terms of crowd control. Dgns were designed to put in effort in even the littlest of things like watching your footing, mobs not clustered together, etc. In my opinion, all this was busywork, to give the sense that jobs had depth...and it sort of worked but at the same time it didn't. If FFXIV back then promoted playing whatever you wanted, just like today, then it was doing a poor job at that. For example, a level 60 cleric without swiftcast was frowned upon. A dps without bloodbath or raging strikes, same deal. It was always a matter of "why didn't you pick up X to do Y?" so in the end, you--and the game--ended falling in the same illusionary pit trap that games often fell in: following the meta.
And I think YoshiP realized that, which leads me to think why he moved away from that. It's why we moved from "old school" FFXIV to "modern" FFXIV. And many a player often conflate that move with the current "mediocrity" you speak of. It's the usual wheel of thought behind convinience begetting laziness. You know, the whole "back in my day, we had to do X to do Y and it made us stronger for it." Problem is, rarely do people on that side of the equation care to admit that they also had dum dums in their time, preferring to hide behind the ideology that because things were harder, you had less dum dums. As if people were not shielding themselves from criticism back then. As if people were not messing up rotations and positional back then. As if people didn't cry foul at the first sign of being called out back then. It's almost like the Tales of DF for the duration of ARR, HW and SB did not exist and somehow, by magical happenstance, it reared its head up only during SHB and EW. Nope, clearly these mediocre players only showed up in droves during the latter expansions and now that most of the measuring skill stick is gone, it's absence has allowed these players to go full bad instead of only minimal bad.
So what's there to be done? Depends. How much apathy can you generate? Can you relegate your performance expectations to areas where it matters like Savage/Ex? Can you substract actual performance level required for an instance you know inside out minus the optimized performance level for said instance? If one is perpetually disappointed with the playerbase in general, do you recluse yourself and finally decide to only do content with your peers of similar talent? Do you use NN to hardline sprouts so you can personally sigh a breath of relief so they won't contaminate other people's future playtime (and of course yours as well)?
Does 14 promote a Culture of Mediocrity? I believe the question already has its answer with you and you simply asked with the aim of wanting relief that you're not alone in this, in your...curmudgeon feelings towards the playerbase in general.