Interesting point. Wavatars are Shamus Young's original work.[1] Identicons[2] and monsterids[3] are Scott Sherill-Mix's work. I don't think those guys were shooting for "ugly", but I agree an ugly or nondescript avatar could incentivize users to upload a custom one. And that may have played into Gravatar's decision to support these avatar types.
Minor correction: Identicons were created by Don Park who open sourced a Java implementation. Scott adapted it for WordPress and gives credit to Don credit for the idea in your link[2]:
"A couple weeks ago I made a WordPress plugin to generate unique monsters for each commenter. Don Park came up with the original idea for representing users with geometric shapes. Since I already had the framework in place I thought I’d make a WordPress plugin for the original geometric Identicons."
They're intentionally ugly. The theory is that an ugly default avatar makes the user more likely to upload their own image.