My boys have always had lots of hair, and I never really paid that much attention to little ones who weren't blessed with lots of hair early on. So I really have no idea whether 18 months is unusual.
You asked the pediatrician about it because you are concerned. That seems like a great place to begin. Good for you. Your pediatrician couldn't give you an answer and suggestion you see someone who could ... a dermatologist.
Don't be nervous just because the ped is sending you to a dermatologist. How else are you going to get answers?
Try not to get nervous anytime a doctor sends you to a specialist. That is simply the doctor being responsible. Peds can handle many, many things, but some things really are better handled by specialists. No reason to worry! Just be glad your ped has your back.
ETA - You also asked how to deal with rude people. This is a skill all parents need from time to time. You have to learn how to not care. People can be rude, and people can be judgemental. You have to learn to focus on your daughter and not care what other people think.
I have a 9 year old son with Autism. One of the best lessons he has taught me is to stop noticing the people who stare or criticize or are just not supportive. As I've learned to do that, I've also noticed just how many people are nice, polite, supportive and genuinely positive.
It is an important parenting skill, and it is an important life skill. You don't have time to worry about people being rude or asking you why your daughter doesn't have hair (as if you have any control over that ... unless you choose to shave her head :-) You just have to learn to not care.