Q: How can I guide my 5-year-old daughter's dressing style without being too critical? How can I discourage her from wearing something (ex. a giant bow on the top of her head) that may provoke a negative reaction from others without my hurting her feelings and damaging her self esteem?
This is a difficult balancing act. You want to encourage your little fashionista's sense of style, but you don't want her feelings to get hurt. But guess what? The strongest source of feedback she will get will be from friends. And you can't shelter her from that. Don't worry about damaging her self esteem: If she's a reasonably well-adjusted child, a little social feedback from peers is actually a good thing, and will build her resilience.
You can casually ask her what she thinks others might think of her outfits ahead of time, to build her internal sense of self-evaluation. But let her decide -- as much as is reasonable -- because her own experiences will have the most impact.
You can, however, put your foot down when the occasion demands a certain kind of outfit. If she needs to wear a jacket to go outside, or if she can't wear her fairy wings to school, or if she needs a clean, simple dress for church -- set those limits. But reserve your feedback on her outfits for the occasions that really matter.