I have no idea what people see in this, unless it really is the case that simply making the lead evil is considered genius by virtually everybody.
There is barely a plot, there is absolutely no character development, what happens is frequently driven by the lead or his minions simply overpowering anything in their path or by his minions figuring things out but assuming that the MC was five steps ahead of them the whole time - a lame plot device that would be funny if deployed sparingly but here it gets old really fast through frequent overuse.
The lead is as OP an MC as any, which requires finesse to be interesting, but here he's actually not very bright and so there really is nothing to be interested in, as I say most problems are simply brute-forced through or his minions figure it out for him.
Being evil (Oh, the genius of having an evil MC!) he does some despicable things, committing murder and ordering slavery and a teensy bit of genocide too. Well, clearly, given that he was just some sort of middle manager last week this must call for some serious soul-searching or PTSD or something, right? Nope. He's an evil lich now so all is a-ok, he can simply send his arch-demon #2 off to conduct breeding and torture experiments on thousands of captives and not miss a wink of sleep. Any possibility of serous commentary, character development or storytelling is completely ignored.
In fact any time the MC starts getting emotional, some unknown effect literally kicks in and turns his emotions off. So that a modern guy who is a product of a modern educational and ethical system can go straight back to ordering the deaths of thousands of people without having to ponder for a second if this is the best way of doing things. What fabulous writing.
Let's talk about the supporting cast: they all worship the lead, literally, are completely sycophantic and totally uninteresting. But they're evil, so they must be the best characters ever. No. They're boring as hell.
Let's talk about how and why he was isekai'ed. No let's not, because Overlord doesn't care about that stuff. We're here to be unremittingly evil for no good reason so you should just lap it up.
The character designs are about the only redeeming feature, the CGI is legendarily terrible, everything is grindingly dark and boring. The first season finishes off with the MC having to fight one of his absurdly strong minions himself - we're now in season 4 and we still don't know why this happens other than MC's speculation that a MacGuffin of some kind is responsible. The fight is settled by the MC using other MacGuffins in his possession - and which he owns because as a player he simply purchased them, because the game he was playing was explicitly as pay-to-win as they come (so the whole crappy mess is ironically originating from a crap PTW game).
If you took everything that people think is crap about bland generic isekai - girls who do nothing but fawn over the lead, lack of threat because MC is too overpowering, poor world-building, no elaboration of the 'game' systems, no exploration of why the isekai event happened, no examination of the consequences of actions - this has all of them. But without the fanservice.
Basically, it's rubbish.
