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.
[1] http://www.shamusyoung.com/twentysidedtale/?p=1462
[2] http://scott.sherrillmix.com/blog/blogger/wp_identicon/
[3] http://scott.sherrillmix.com/blog/blogger/wp_monsterid/