I, too, defintely agree with Chris and Adrian.  If there is an experience UI or UX designer then let them go for it...that is their specialty.  However, if one is not available then I think a BA and BSA should do UI mockups.  Mockups are not set in stone and this needs to be related to developers so they know that is simply a guideline to give ideas.  I have ran into too many BAs who will not do UI mockups when there is not an UI (or UX) designer simply because they state it is not their job.  I think this is laziness and it actually hurts the business owner.  I truly believe that BAs should know some degree on how to create mockups for UIs.  I have also found that with some of the BAs I have worked with lately not collaborating with the development team.  I just cringe when this happens.  Our job as BAs and BSAs is to collaborate with various groups and people. 

To reiterate what Adrian stated, I think if a BA does not have UI experience then try to get training or work with people you know to help you with learning how to do UI mockups.  I would also recommend to understand and learn the analysis with eye movement, hand movement, wording, etc. since these are all critical aspects to understand when creating a mockup and useability.  It also can increase the time an end user takes to do a job function or it can decrease the time.  Once a mockup is completed, then I would meet with the business owners to discuss the layout, clarity, and color of the UI.