That's probably overstating it; it is definitely true that his redesign of iOS 7 (2013) was truly awful - he really should not have been anywhere near software, and was able to take advantage of Scott Forstall's then bad reputation and recent dismissal to insert himself in an area he had no real qualifications for. Before that, he was purely a hardware guy and it was clear from the outset he was in over his head with software. Apparently he had no idea what 'HCI' (human computer interaction) even meant; wasn't even familiar with the abbreviation itself. Now with the benefit of hindsight, we can judge that iOS 7 had some truly terrible design decisions that ended up making the iPhone much harder to use (very light, unbolded text everywhere, removal of borders on everything especially buttons, over use of color tints on text).
What I always found crazy was that Ive seemed to just take design ideas from then Windows 8 and Windows Phone more than trying to create his own thing. It showed that he had no original ideas of his own; even just iteratively improving on iOS 6 would've been better.
On Mac hardware, he definitely needed some sort of editor to stay his hand post Jobs. The era of crappy butterfly Macbook keyboards is still something I remember that was clearly his responsibility, driven by obsession on thinness and it seemed for a while that Apple was in denial about the issue.
Still, the Apple Watch is a definite hit for Apple now and it's clearly his baby, so his legacy isn't all bad.