Dark Souls is pretty great actually. I played through it last year and had a blast. Even the second half, which is generally considered worse (rightfully so) wasn't that rough. I quite enjoyed the much-maligned Tomb of Giants, for example - great atmosphere.
The world design is excellent. Lots of hidden areas, many places are interconnected like a metroidvania, usually there's a sealed gate you can only open from one side which starts to feel like a gimmick after a while but it works well enough for the purpose it has. There's a large variety of locations and they're all densely atmospheric. Your claim that it's all just dark and gloomy and looks like shit isn't even true, the first proper area you enter is a medieval castle with daylight, later on you enter the bright city of Anor Londo, there's a gloomy forest with plenty of green, and Lost Izalith adds a little red with its lava streams. And when we look at Dark Souls 2, it gets even more colorful, beginning with the starting town which is a beautiful and serene place, and pretty much going through the entire game which boasts great diversity of locations.
The gloomiest-looking Dark Souls game is 3, and I'd honestly consider that one the worst of the series. More linear level design with fewer interconnections and backtracking, and a greater focus on choreographed boss fights that are way too hard for their own good (many DS3 bosses took me 20+ attempts).
But DS1 and 2 are excellent games if you're an explorefag, as is Elden Ring.
The genre gave birth to some other quite good games, too. It has some features I dislike (losing your XP on death and having to do a corpse run to get it back fucking sucks), but the exploration-focused gameplay is top tier.
The only soulslikes that suck are the ones that embraced the difficulty meme too hard and overfocus on boss fights above everything else. Lies of P receives a lot of praise but from what I've read, it is far more linear than any Souls game and its main focus is on the combat, particularly boss fights, rather than exploration. A fundamental misunderstanding of what soulslikes are about.