毕竟我是个很哇塞的大萌比毕竟她是个很哇塞的大萌比
[bì jìng wŏ shì gè hĕn wā sài de dà méng bĭ bì jìng tā shì gè hĕn wā sài de dà méng bĭ]
Here both '哇塞' and '萌比' are internet slang which mean being amazed and extremely cute respectively, so this nickname roughly means 'anyhow I am adorable'. Such nicknames often used to represent their own loveliness playfully, while using twice shows they are very positive on that.