it's more of an art direction issue than anything technical, there are PS2 games that look better than WW. I'd agree that the sky in SS isn't an improvement though, while I do think SS has much better art direction and has a better sense of fantasy throughout, there are a lot of times when I'm playing it where I really have to question nintendo's knack for making games at this point, there's so many little things that just shouldn't happen(treasure messages, insanely slow text speed, travel STILL being an issue, arbitrary stamina bar and random use of motion for things that really didn't need it). it's weird that they do things like letting you run and leap while clinging in an attempt to speed the game up but then do so many things to slow it down, SS really should have been the best game in the series but they did so much to make it a grind, it's like there was no unity in it's design philosophy at all, just a lot of different people doing their own thing with seemingly no communication between them to make sure everyone was on the same page
When I see how good Mario Galaxy 1 and 2 were, it's clear that they have a lot of talent still and can make good games. NSMBWii was really good too. But if you compare SS to the 2D Zeldas for instance, it's clear how different the design philosophies were now and then. At least for Zelda.