Q:

Is there a way to flag offensive usernames? 

I'm pretty liberal, but sometimes users pick rather offensive usernames. 
Unlike questions and answers, which have a "flag" link where you can mark them offensive, I couldn't find a similar functionality on the user's home page. 
Is there a way to do this that I missed? 

A:

It has already been flagged, and the user has been notified.  It will be changed within the next 12 hours, one way or another. 

A:

Just for the sake of completeness I've only seen it rarely on Stack Exchange sites but there are a couple of ways an offensive usernames can become visible without them making a post:

They make a reasonable suggested edit as user1234 that's accepted and then go back and update their profile with an inappropriate username.  Then it appears on the main page as the person who last modified it. 
They pick a username that people are likely to search on such as a moderator or high-rep user with something that ends "is a ..." or similar on the end. 

In those cases you can flag either the post they edited or any post of your own for the latter with "other (needs ♦ moderator attention)" with a description of the problem.  I only remember doing it once but make it easy for them and include a full description in the message such as:
This doesn't relate to this post, but the username https://electronics.stackexchange.com/users/1/offensive seems offensive and they have just made an approved suggested edit that makes the name visible.