What ruined the manga was Naruto himself and the whole Bijuus and Jins. In order to keep the playing field equal, Kishimoto relentlessly upped his game with every antagonist he introduced. This became so laughable by the time Bijuu Dama was introduced that I laughed my head off, honestly.
Had Kishimoto kept them as "chakra monsters" rather than monsters with "chakra flesh" things wouldn't have been so blown out of proportion. It was so damned hilarious to watch that logic being broken apart and pieced together to convince us that they are still chakra monsters. Chakra was turned into physical entity beyond the point of no return. The author had to drag everyone up to this level of laughable, scifi, nuke-level powers so that dear old Naruto wouldn’t be left alone; he had to be coddled.
Couple that up with Obito, who ruined it completely with his typically embarrassing thoughts on how he thinks “Naruto and him are the same.” How many people are the same as Naruto in this manga, again? I think I have lost the count. His childish tussle with Naruto on childishly stupid ideologies centering on "Will of Fire and How to become a Hokage (your very own handbook guide)" were nauseating and the stuff of one of the worst writing I have seen in any medium. It was laughable. Who were they appealing to? 5 year olds or people that cling to those bygone childhood days? Silly.
Every antagonist that came across him took on something as if being infected with a lethal pathogen that had one mission: destroy that character from the inside until not even its husk remains. Tobi was one of the many causalities that would shock any decent reader’s sensibilities.
They (Kishimoto and Co.) should have quarantined him, given him limits. But, nooo. That would have been the sensible thing to do. The only saving grace of this manga was Sasuke, Sharingan, Uchiha dynamics and few other threads. This manga was ruined beyond redemption from the moment Nagato decided not to kill Naruto and take the helm of the idiot ship again. We saw flashes of good narrative with the Kage Summit and Danzo arc. The rest of it? I kept hoping something good will come of it … and the manga ended and that lingering feeling of being lead on is still there.
So no, the manga was mediocre long before it. It simply plummeted below it immediately after.