Saying that it is "crappy" is not very explanatory, is it? It is playful, maybe a little childish, but it is consistent (grub, gdm3 themes, desktop wallpapers, etc.), and high quality artwork. I have no clue why it was actually chosen, but my guess would be "universal operating system" taken kind of literally.
If you do not like it, there is an easy fix:
Check /usr/share/images/desktop-base/ for what these images actually look like.
sudo update-alternatives --config desktop-background
sudo update-alternatives --config desktop-grub
sudo update-alternatives --config desktop-splash
Also, if you are running plymouth:
And if you are using gdm3, you can set your favourite picture in /usr/share/gdm/greeter-config/10_desktop-base.
sudo plymouth-set-default-theme -l
sudo plymouth-set-default-theme $ONE_OF_LISTED_ABOVE
sudo update-initramfs -u -k all
Lenny look is still available in the above choices. Enjoy you custom Debian look!