It significantly impacts the early part of any relationship, but it's not everything, and can actually be bypassed to a certain extent, but this requires the person in question to not be truly noticed at first (such as a person making a comment/joke that draws you into a conversation). There are also certain boundaries of comfort that each person has (that can be exceed both by being around a very unattractive person as well as someone who is much more attractive than you are normally used to being around), but there are a lot more factors than just visual ones that are involved in the first judgment process (including smell, voice, movement, expressions, etc.).
You can get used to a person over time, and eventually befriend them, but probably this means that a romantic relationship will not happen, or that there could be future issues if it does (remember, I'm also talking about people you deem too attractive as well, as this can mess with your mind).
No looks aren't everything, but yes they are important.