I think it probably is being done on purpose. And probably for a specific purpose.
For one thing, hyping up Rocks is also hyping up Blackbeard. Since, they're obviously so related. Making Rocks seem super strong, makes us associate that same strength with Blackbeard. We really haven't gotten to see Blackbeard actually do that much in the manga. So, having Blackbeard's predecessor do all these amazing feats, kind of also makes the audience associate those feats with Blackbeard. It's all in service to build up to Blackbeard finally making his big move in the manga.
And secondly...it's probably also showing us why and how Rocks failed. Rocks seems to have built his crew in order to entirely maximize power. And that's probably the "wrong" way to build your crew, if you want to find the One Piece. Rocks has all the power in the world that he should need in order to succeed. But, he still failed. I think this is Oda subtly showing us that raw power isn't enough for someone to claim the One Piece. You also need something else that Rocks didn't have. Something that Luffy does have.
In my opinion, a lot of this manga is Oda criticizing different "JRPG Party Builds". You kind of have to think of One Piece like a video game or a RPG. You have a set of characters in your adventuring party, and how you let them grow and what they specialize in changes how well you can tackle the game. If you mess up how you let your character's grow, you could "softlock" yourself and make it so you're unable to finish the game.
Rocks, much like Blackbeard, is basically going all in on Devil Fruits. Almost his entire crew has Devil Fruits. That's kind of making your entire party Black Mages. It's a strategy you CAN use, but it's not the best way to beat the game. If you hit a enemy that's strong against Black Mages/Devil Fruit Users, it can wipe your entire party and you lose.
Shanks is terribly strong, but he lets a lot of weaker people into his crew. That's kind of like ONLY ever using your Main Character/Paladin the entire game, so he's overleveled and can basically solo anything he comes across. Shanks is level 90, when everyone else is only level 60. That's also a strategy you can use, but again, if something DOES take out your Paladin, that means you really don't have any other characters strong enough to fight back.
Buggy...basically dumped all his stats into "Charisma" and "Luck". So, he's terribly weak, but he's able to get others to work with him fairly easily. Buggy might actually get farther than anyone else, just because he's managed to luck into recruiting some powerful people along with some very loyal people. We'll see how this strategy works out.
Even Roger was kind of "playing the game" wrong. He was speed running to get to Laugh Tale, but he didn't do any of the Poneglyph "side quests". Which means, he had to "backtrack" all over the world a second time, and he basically ran out of time, due to his illness.
Luffy's crew is probably going to be a example of "playing the game" correctly. Luffy has a "balanced" crew. You have strong Fighters, like Zoro and Sanji. You have some Devil Fruit Users, like Chopper and Brook. You have some weaker characters, but they have specialized skills, like Nami and Usopp. You have certain characters with a lot of knowledge in specialized areas, like Robin and Franky. And Jinbe...can swim good. It's a good mix for facing down a wide variety of enemies. Most of the Strawhats are strong, but more importantly, they all have specialties outside of their strength that make them valuable outside of battle. And they've been taking their time. Doing a bunch of "side quests" in order to level up and collecting all the Poneglyphs along the way, so they won't have to "backtrack" to collect everything at the end of the "game".
So, yeah, two main things. It's giving Blackbeard some hype, by association. And it's showing us that strength alone isn't enough to succeed on this particular adventure. You also need something else. What that is exactly, we probably won't know until the end. But, I'm betting that's what Oda's going for. Rocks failed for a reason, and that reason probably has to do with how he chose to construct his crew around strength above anything else.