It is a adjective that is derived from "薄い". "薄い" is used when describing something that is thin or when the color/taste of something is light/mild.
”薄っぺらい” is used when describing a shallow person or something that has superficial quality.
Tom and Jim are complaining about their Japanese language class.
- Tom: I don't want to take the class anymore. It's too boring.
もう、あの授業を受けたくないよ。つまんな過ぎる。 - Jim: Yeah, he just goes by the textbook. His lessons are devoid of any substance. I can learn what he teaches on my own.
うん、あの先生さ、教科書を読んでいるだけだよね。薄っぺらい。自分で勉強できるよ。
Ken and Leo are talking about what kind of girls they like.
- Leo: What kind of girls would you like to go out with?
どんな女性と付き合いたい？ - Ken: I like girls who are kind and sexy. Oh, they have to be thick.
優しくて、セクシーな子がいいな。あっ、スタイルがいい子ね。 - Leo: Huh, you are so shallow.
はーっ、薄っぺらいなー。
Meg and Amy are talking about personal relationships.
- Meg: As I get older, I find that having a few close friends is much more valuable than superficial relationships with many people.
年を重ねると、大勢との薄っぺらい関係より、数人の親しい人との関係が大切だってことに気付かされる。 - Amy: What happened all of a sudden?
いきなり、どうしたの？ - Meg: No matter what community I am in, I feel alone sometimes. I don't know, maybe I'm just tired.
どのコミュニティにいても、時々孤独を感じるんだよね。よく分からないけど、多分私疲れているのね。