Jiren is far from the strongest mortal. Remember, there were four universes considered too powerful to participate in the tournament of power. Jiren and Goku are bottom tier compared to whoever is the strongest out of the aforementioned universes.
It doesn’t matter how hard Jiren trains. At this point, Goku is close to naturally surpassing him and Vegeta will follow. As saiyans, they are biologically designed to become stronger with every battle. Even as powerful as Jiren has become, he has a limit. We’ve seen that the saiyans do not. Much like Hit, Jiren will take a back seat to their evolution.
The only opponent that’s ever truly bounced back has been Frieza. While it’s mostly a plot device, it makes sense that he would still be able to push his limits, as he rarely ever has. How long that will last is uncertain. I don’t see him being a major villain anymore though. Goku and Vegeta still appear to be well above him.
Jiren was a cool quasi-villain, but he certainly isn’t the be-all-end-all he was made out to be. He’s a very flawed character in the end.