There's different "levels" of dressing well. For example, with respect to point 1, not everybody who dresses well puts a lot of effort into it or looks like they put a lot of effort into it. Somebody can dress very simply and effortlessly and be considered well dressed. Somebody else might put a lot of obvious attention into how they dress and look awful (i.e., a "fashion victim"). (To make matters more confusing you can spend $2000+ on a t-shirt, jeans and canvas shoes. So you can look casually and simply dressed but be very self-conscious about it.) There's also the difference between looking like a slob and exhibiting some sort of care and attention about how you look. In certain situations looking like a slob can be taken as a sign of hostility (i.e., showing up to a meeting or formal gathering looking disheveled). It's also usually possible to tell the difference between someone who is indifferent to their appearance (they might still be considered attractive, for example) and somebody who is an incompetent dresser (i.e., they've made an effort but it's all wrong). People often spend a lot of time trying to signal apparent indifference (this is common among male celebrities).
I think if the way you dress sends negative signals, you probably want to address it. Figuring out whether that's the case may not be easy.
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.