It is definitely not a bad Pokemon, but I can see why this is happening. While it threatens set-up sweepers and practically makes them unviable, it doesn't do so well against Choice item users or common stall Pokemon. The best it can do to Pokemon such as Slowbro is use their status move of choice against them. It also can only speed tie Choice Scarf users, while Choice Band/Specs users can't have their item boost stolen. Specs Keldeo, for instance, can OHKO Ditto with Secret Sword if it took some residual damage, while Ditto does around 55% with Secret Sword. It also. unfortunately, can't trap Chandelure locked into Overheat or something besides Shadow Ball.
So yeah, while it makes set-up sweepers run screaming, it matches up not so well with the other 90% of the metagame. Even so, I still think Ditto is worthy of a teamslot if your team struggles with set-up sweepers.
[22:11] Helias: FUCK YOU HAWKSTAR THAT'S NOT MY NAME