I wonder if Peridot doesn't consider herself an alien because Earth was originally supposed to belong to Homeworld. Even though there are tons of other living beings there, Homeworld still considered it theirs and Peridot believed that. So even though she's a new visitor to Earth, does she realize that she's the odd one out? Spending so much time with the Crystal Gems and not really traveling, it might be difficult for her to comprehend just how vast the planet is and how many different creatures live on it. She's learning.