Boss fights are a test of how well you know your things the same way that a literal college test should. The boss/test should be harder than base assignments/mobs, but not too difficult to pull off either due to lack of familiarity intricate knowledge of a system, or knowledge of a specific tool needed for this boss. Oftentimes, with preparedness, the boss can be as trivial as the mobs with your level of skill and preparation, but should maintain a base level of difficulty
Since OP posted DaS 1 I will give two examples, one of a good boss and one of a bad boss. Everyone knows that the bed of chaos is shit. But from an academic standpoint it ticks every marker of what a bad boss entails:
1) the difficulty is not about expanding upon knowledge you learned throughout the previous portions of dark souls. The BoC is trivial to dodge its direct damage, roll spamming works 95% of the time, especially in the first segment, there is no health gage that you need to worry about with the BoC.
2) the difficulty is random: its basic attacks are telegraphed but the problem with the BoC is its pushbox that stiff affects you according to the geometry of the branches. Unfortunately DaS' collision meshes and geometry is dogshit and when you are pushed, it is up to pure chance to determine wither or not you will survive. Combine that with the pits that crumble and you can get pushed into the instadeath pits even if you dodgeroll perfectly from its swipe.
3) you are meant to lose the first time: most casuals will say that dark souls is trial and error when it comes to boss patterns. However someone gud enough can dodge most patterns or stay far enough away to learn the patterns. Due to the aforementioned randomness, however. A player who is encountering the BoC will die the first time they enter the arena. The areas of crumbling ground seem logical in retrospect but without the knowledge that the ground will crumble in the first place it's expected that someone will walk right into the holes he created by shooting/hitting the first root.
4) trivialization: the bed of chaos is utterly trivial due to two factors. One is simply waiting for a good pattern and rushing the next objective, destroying any semblance of skillful play and reducing it into a game of "how many times will I die before I get good RNG" the other way to trivialize it is to look up a guide and figure out where to shoot an arrow. Unlike a specific cheese build meant to poisebreak major bosses like Iron Colossus literally all you need to do is grab a bow and shoot in a specific spot pointed out by a fucktard slav on youtube.
Good boss: Artorious
builds on skill: you need to learn how to get damage in, get out of retaliation range. Know when to sideroll, frontroll and backroll based on attack patterns, estus and attack timing is vital. Telegraphs are clear and noticeable but quick.
Steady difficulty: all his moves are difficult, a few being easier to dodge and having longer cooldowns to give you enough time to chug estus, apply buff or swing a few times. you can still go hyperaggressive and poke between quick recovery attacks you are very confident
nontrivial: despite the static difficulty, the pace of the fighting and raw damage a boosted artorious can deal means that pretty much any build is in danger of dying if a couple of hits connect, especially after his buff. If you can dps enough to poisebreak his buff charge you can get a lot of damage in but it is no means a trivialization of the fight, it's a high risk high reward strategy since if you fail and you're within explosion range, you're almost guaranteed to die to the blast.