As a Jungler, I agree, I have never had a problem with Lee Sin, outside of the fact I've always felt he has a tad bit too much mobility and the AD scaling on his ult is excessive. The AD scaling is hardly a huge priority though I would argue.
But considering his identity and the way his early game pressure works, that's fine by me, I would argue he actually needs the hyper mobility. People laugh at the notion that he falls off hard in the end, but he seriously does. By the time end game comes, his only advantage on the field of battle is his hyper mobility. I don't know a single jungler who falls off harder then Lee Sin by mid-late to late game. By the time that time of the game comes, his presence on the field is laughable at best assuming you didn't feed him (but the same goes for literally any other champion, so that's a moot point).
If he tries to build AD to take advantage of the insane ratio he has, he's going to die before he can even get to your carry, and if he builds tanky, he's not going to do all that much damage to begin with.
I would say its not that Lee Sin needs to be tone down, but Lee Sin should be the example of a good champion. He looks and feels powerful, but he's quite balanced outside of a few minor issues. The issue with Lee Sin doesn't seem to be that he's THE best Jungler in every meta, it has more to do with how when the meta changes, certain junglers are no longer viable whereas some become stronger, but Lee Sin will always be reliable, no matter what happens in the jungle.
But isn't that what we should strive for every single jungler? Lee Sin is the standard, but he's the standard by what junglers ought to be: viable regardless of what happens in the jungle. The job of Riot isn't to bring Lee Sin down, its to bring all the junglers up to his level.
But lord knows the ineptitude which Riot possesses with stuff like that.