> cartoon figures in professional applications (please explain to me how Octocat is different from Clippit)
Is Octocat not a cartoon figure? Is Github not a professional application?
I'm not aware of any authoritative definition of user experience that excludes "aesthetic grounds" from the field of UX. How an interface looks and the feeling it conveys is a part of the user experience. The UX industry doesn't operate with this distinction -- see e.g. NN Group's oft-quoted definition (https://www.nngroup.com/articles/definition-user-experience/), which considers aesthetics, and UI design in general a component of UX, or the ISO 9241 definition.
A tongue-in-cheek logo is one thing, but a goofy interface, from the ever-present logo to the overall widget design, is way out there in kindergarten land. The comparison between Octocat's use and, say, that of the Rust crab, is itself unwarranted: you can (in fact, I do) write Rust all the time and not see the crab for weeks, whereas that goofy cat critter is literally on every Github page.
As for Clippit specifically, while its graphical design was obviously not the primary reason why it was hated, it was definitely one component of it -- in fact, it's one of the reasons why it was specifically considered distracting. Contemporary studies, like Swartz's notorious dissertation about why everyone hated Clippit, obviously focused on the functional aspect, but it's telling that the quantitative framework in use at the time put "fun" and "useful" in separate categories.
>you got to look at a goofy, childish character
Like I said, aesthetic grounds.
>whether you wanted to write a cute check from the tooth fairy for your niee, a report on inappropriate use of company resources, or a death notice for your late father's friends.
>You could at least turn it off, whereas Octocat is in the top-left corner of literally every Github page, and prominently featured wearing an astronaut's costume (!?) on the homepage.
This is quite an inane complaint. Plenty of serious products and companies have cartoon characters as mascots or logos. LLVM has a dragon in the logo; the Go language has a gopher; the Rust language has a crab; the FreeBSD and OpenBSD OSs have a devil and a blowfish respectively. All of these have varying levels of visibility when you visit their web sites.