What I've read of the psychology literature generally indicates that mirroring the dress sense and behavior of your target audience gets you further than adhering to some codified notion of respectability when you're trying to sell something, and that this remains true when your product is a religion.

When what you're trying to sell is status-linked, it can be useful to act one or two status levels above your target audience. But there's no clear link to status here, so I'd imagine the Mormon uniform has more to do either with intra-group signaling or with an attempt at mirroring a large cross-section of potential recruits that became fossilized sometime in the past.