In this visualization, I've tried to group the various Mojo classes by function and color code them. The classes are placed in layers, and related groups are close to each other. Note that I've dropped abstract base classes and Mojolicious related subclasses from this visualization, as well as some servers and all the commands.
This might be useful as a quick reference, if you want to print it, you can get the PDF here.
I've stuffed the omnigraffle source as well as a pdf and jpeg representation on github, in case someone wants to play with it:
https://github.com/marcusramberg/mojo-classes. It's all licensed under CC BY-SA, so feel free to hack on it.
*updated* Was missing Test::Mojo, also made it a bit more symetrical while I was at it.