The design for the cactus in the desert portion of Super Nintendo World looks a lot like a fan-made asset, but that may be because it's one of the first search results when looking up "New Super Mario Bros. Wii Cactus" on Google. It's likely an accident, but the modders don't mind.
In the picture revealed by Nintendo, you can see a cactus alongside a Pokey, those tall spikey enemies made up of several balls stacked on top of one another, and this cacti design doesn't actually appear in any Mario game - at least, not officially.
Meatball132, who is working on a remastered version of the mod, pointed this out on Twitter. They reached out to some of the original developers who also agree that Nintendo likely based it on the mod. However, they added, "I just want to be very clear: it wasn't my intention to make a big deal out of it. It's just a cool thing that we noticed and wanted to point out. Don't get angry at anyone for this, please."
Nintendo actually using something made by fans rather than shutting it down and threatening legal action is a nice change of pace, although it seems as though it found the cactus through a google search, so it might not have even realized at the time that the cactus wasn't from the official game. Either way, the outcome is the same.
Sure, cactus' can't look that different but the design is an exact replica, from the positioning of its arms to the flower. The only real difference is that the flower on the cactus is yellow and not pink, but barring that, it's the same shape, in the same pose, and around the same height and color. It's a cool oddity, a little goof on Nintendo's part, and something that the developers aren't fussed about, but it's a fun observation all the same.