It is. It has the same problems with everyone having different UI tweaks, but at least the base Android comes with _some_ UI. Fragmentation isn't that big of a problem for users, because there's only so much you can do on a phone - between your photo album, contacts app, web browser and social media, any kind of UI will do. It also helps that most popular apps ship their own UI.
Android took off when we had HTC sense, Samsung touchwiz and others, almost no one was running the stock UI. But if KDE and GNOME and others were all running on GTK you'd still probably call it fragmented.
There's no consistency between apps on android so you don't notice it as much.
