This post commits the relation projection fallacy with respect to the word "signal," which isn't a two-place predicate but a three-place predicate (at least): the well-formed sentence isn't "X signals Y" but "X signals Y to audience Z." Two values of Z that might be relevant to LessWrongers are "rationalists" and "most people," and there's no reason to expect the answers to be similar in the two cases. For Z = "most people," options 1 and 3 in particular strike me as wishful thinking.
Barack Obama is an example of someone whose time is much better spent not thinking about what to wear, but that doesn't imply that he dresses poorly (because he also recognizes the obvious importance of his dress as a signal):
You need to remove from your life the day-to-day problems that absorb most people for meaningful parts of their day. “You’ll see I wear only gray or blue suits,” he said. “I’m trying to pare down decisions. I don’t want to make decisions about what I’m eating or wearing. Because I have too many other decisions to make.” He mentioned research that shows the simple act of making decisions degrades one’s ability to make further decisions. It’s why shopping is so exhausting. “You need to focus your decision-making energy. You need to routinize yourself. You can’t be going through the day distracted by trivia.
Yesterday I attended a meetup where the discussion turned to fashion for a time (because apparently the mini-camp participants were given some instructions on fashion as a useful part of instrumental rationality). (Unfortunately none of us knew much about the topic so the discussion turned into "how can we find an expert to advise us for minimal cost?") It was mentioned that dressing "badly" can be a useful signalling device, and some examples were given. Here's an attempt at a more complete list of possible signals one might be sending by dressing "badly".
The idea here is, if you do decide to start dressing "well", know what you're giving up first. (Of course you're also giving up possibly implying that nobody taught you how to dress and you're not sufficiently strategic to have thought of learning it yourself. Or implying that you don't have the mental, financial, and/or social resources to keep up with fashion. A lot of signaling depends on what your audience already knows about you, or can infer from your other signals.) See also Yvain's related post, Why Real Men Wear Pink and comments there.