They lost the copyright of the Steamboat Willie characters as well. So the Steamboat willie version of the Mickey Mouse design is in the public domain. All the other Mickey Mouse designs are still under Disney’s copyright and they still own the Mickey Mouse trademark.
So yes you can use Steamboat Willie Mickey in anything you want and create derivative work.
They still have a trademark on Mickey mouse, so it's not carte blanche. What extent the trade mark has isn't perfectly clear, but it's pretty powerful.
Not to be pedantic, but that doesn’t resemble the Steamboat Willie version, which is the one in the public domain. That looks more like modern Mickey, which is still under copyright. The SW version has smaller ears and a longer nose.
Yeah, a lot of people are missing the point of this whole thing. Only the 1928 Steamboat Willie version of the character is in public domain. Most of these AI-generated examples contain features of later versions of Mickey (in particular the later, rounder head shape which is more like Fantasia-era Mickey), so these would likely still fall under copyright.