In this post, Witherington makes the case that the appearance and demeanor of the preacher matters. Some excerpts:
As the rhetoricians will tell you, you need to establish rapport with your audience at the outset, and if you have some ‘inhibiting’ factors to establishing that positive rapport, then you need to overcome your ethos liabilities.
At a minimum he should be clean and neat. Preferably he should look like what he is about to do actually matters. ‘Casual is as casual does’ Forest Gump might say. But there is nothing casual, off-hand or simple about the Gospel and its proclamation. While I am at it, there is nothing casual about worship.
I am saying we should come looking our best, whatever that is, whether simple attire is the best we have, or formal attire is the best we have. We should give God our best.
Being a small group leader may not be as public and there are fewer eyes on the facilitator, nonetheless, it is important that she/he take that responsibility seriously. Thus, paying some attention to your appearance and demeanor sets a good tone for the group.