So, i'm reading part 6 right now, and i must say, as the time goes on, Jotaro gets more and more nerfed.
To be fair, after part 3, even with time limit abilities, not only he got nerfed, he isn't as strong, smart or with some smart decisions, he was just, there, like, after part 3 ended he was the most powerful stand user (At that time) and he got a little of renown in Part 4, but Part 5 & 6 are different stories.
Ironically, out of all the Jojo's, accept it or not, the fact it's that Giorno is the most powerful jojo out there, and many people aren't giving him the "Oh my god he is so cool" because GER, and while i really like his stand and character, GER is a broken stand, probably even more than Tusk (ACTIV), but outside of Giorno, remember that after Part 3 the effects of the stand were more complex.
For example, Magician's Red or Silver Chariot were easy to tell what they do, but stands like Killer Queen Bites the Dust, Echoes, King Crimson, that alien dude from part 4 that is still not confirmed if it was a Stand or he was an alien, etc.
So, by taking those examples, Jotaro's Stand got weaker, not because he isn't powerful, it's because the effects of new stands were more approachable, complex, and harder but at the same time useful to use, Star Platinum is just a machine of punches with time limit included, you really can't do much outside of punching and time stopping for what, 5, 6 seconds?.
And also because he was becoming older and older, he didn't trained too much so you have to take that into consideration, to be fair, if he was more perceptive, maybe if he decided to train seriously after the end of Part 4 and develop all he could with SP, things in Part 6 weren't so messed up, but still, you can't expect much from a Stand who has a development potential of "E" because Star Platinum: The World, which, by Part 6 tier stands, he sucks, yeah, the top tier ability from Part 3 (Time Stop) is now one of the lowest for the same effects of new stands, he couldn't catch up with all of those sadly.