Obviously dramatic over simplification but it doesn’t seem to be the usual kind of thin for the sake of thin design - this iMac seems objectively better than its predecessors and its thickness (or lack thereof) doesn’t seem like a problem or unnecessary constraint.
Again, they don’t have to solve the mobile/tablet issues on this product, it’s never going to be a portable device.
They made a trade-off to make it look uglier/bulkier/worse from the front when actually using the machine, just so that if you happen to glimpse it from the side it's a little thinner. There's not even the argument of portability to support the pursuit of thinness here as with most of their other devices. So I would argue the opposite, this new iMac is just about the pinnacle of thinness for the sake of thin design.
(You could of course argue that it's not ugly, that's totally subjective, but they clearly went out of their way to make it as thin as possible).
They will always be two different use cases, desktop/mobile.