Since Launchpad is an integral part of Ubuntu, would questions relating to the use of Launchpad be on-topic?
Obviously questions about a specific project on Launchpad would be off-topic. (Unless there's another good reason for the question.)
I think they should be considered ontopic.
For more advanced users who might need to follow the progress of a bug or report a bug it can be useful.
And for developers (especially SE people who are unfamiliar with Ubuntu) it's a great opportunity to showcase the things Launchpad can offer them as application developers (daily builds, bug tracking, PPAs, etc.) - which in turn get consumed by Ubuntu users.
I think that aksubuntu.com can provide an user friendly service for learning how Launchpad helps Ubuntu. In the same time, I am not sure if the LP developers are using askubuntu.com.
For better results I suggest using the Launchpad users mailinglist list (I know is not that user friendly ... but rather developer friendly) or ask a question in the „Answers” section for Launchpad itself.