I think get what you mean, let me try and rephrase it a bit.
You have played through the game and you like the game, you had a lot of fun. Your question is, for as much fun as you had and for as long you have played the game, shouldn't you have an easier time against some of the bosses that come later in the game?
Then you can ask yourself if you can only consider yourself "good" in Dark Souls if you have no problem against those bosses after that much time of preperation and if it is your fault or if the game is simply not designed that way.
Being good at one specific thing can mean a lot of things, I believe the majority of people had trouble with many bosses from beginning to end in Dark Souls, thus nobody really was good at it.
I, other players and probably you too have crushed some bosses on the first try, now does that mean you were good at playing Dark Souls? Maybe, I don't really think so myself tho!
Some people had next to no problems at all with anything and that can also be linked to the way they have played the game, but that maybe was so inherently different from what you did that I also wouldn't say they were better at the game than people who had more trouble. They simply leveled up a ton and grinded the game to death (I did this a lot because it was fun to me at the time) or don't engage in direct combat and kill everything with magic, even then they died to bosses later in the game! Now I don't want to say that you don't need to be good at parts of the game to do all these things. But is that what it means when people say "git good" when it comes to Dark Souls?
Here is what I think "being" good at Dark Souls means and it's different from "getting" good:
Being naturally good means you are naturally good at reading enemy behaviour and are able to adjust tactics quickly because you are observant and curious about all and everything in the world around you.
(this was not me, I looked shit up on the Dark Souls wiki)
Still that doesn't mean you wont die a lot (especially not with stuff like the Bed of Chaos or the Four Kings etc), just that you're optimally prepared for the game wether you are even aware of that or not. Being good at that also means you get better the longer you play it, but that's the same with people that are not naturally good at it, you're just faster learning.
So when you are not naturally good at it you learn slower and have a harder time. You can still outmatch people that are naturally good at the game by putting in more effort but that's besides the point.
Most people get good at the game by repetition, there are people that are good at timing and they block/parry and evade better than others. Those people usually will have an easier time with the game, but they also have to learn the different timings and attacks from enemies as they don't know the game when they first play it and so it's not that they are magically prepared for everything the game throws at you.
Then the game has many mechanics to build different characters with different abilities. It's not even meant to be viewed in the way of one perfect playstyle and you are doing it wrong if you play differently. If being good at Dark Souls only means you can read enemy attacks easily from the get go and you have good reflexes then you are dismissing the majority of the game.
For some people it's fun running around naked with a dagger or dressed in robes and a staff casting spells and others like the sound of enemy attacks getting deflected from a heavy shield and shiny armor.
So I think you can be good at Dark Souls and you can get better at it too. It just doesn't mean much as the game itself is not perfect so there is no perfect run on the first try (prolly not the second and third either etc) and as long as you are having fun at what you are doing that's more important that perfecting the game on one mechanic alone or one build (or all builds).
I crushed Gwyn on my first try, doesn't mean anything as I grinded Darkwraiths for days and played around 30 hours simply killing Artorias for other players as a summon. So I had poise up the A due to leveling and items and Artorias shield, lot's of +15 weapons. I was able to block the heaviest attacks from Artorias and still stand and still had stamina to spare. I could block-tank many enemies without magic or Havel armor.
So a lot of the game felt easy to me after that, I still died though (directly to Kalameet after that and quite a few times at that lol) and wouldn't ever consider myself being good at the game in a general sense of reading enmey behaviour easily, because I was honestly terrible at that. I know the game by playing it a lot and getting information on it from outside the game (boss tactics, Smough/Ornstein pillar draw run etc), some people call that cheating, and I somewhat agree on that. I like cheating though
The Important thing was that I had fun playing the game, only when you feel like that gets out of balance and is affecting your enjoyment of the game then you can ask yourself if "getting good" is the answer to that or if there are other ways.