I believe your daughter is just stimulated by her friends into being more animated. That's perfectly normal, and happens to all introverts. I wouldn't be surprised if she needed a lot of downtime to "recover" from such social output, which might explain why she comes off as "grumpy" at home. Some of it might be masking, yes, but that's fine. Masking is not a problem by itself, and there's nothing you can do about it. You need to leave her alone. Also at this age, it's common to be in a constant state of irritation with parents!
My autistic son has a relatively flat affect, which I've never worried about. ADHD meds made it worse, but now he's tapered off his meds. The BIG problem is that he's asocial by nature, which poses a serious threat to career advancement and making friends. Right now at 20, he has no friends at all. He doesn't know how to be interested in others such that he can maintain a friendship. We tried so hard when he was younger, trying to set up playdates and get-togethers, and enrolling him in social skills groups... but it was leading a horse to water. We can't make him drink.