円滑な人間関係＝smooth comunication

It's mature for the people to have smooth communication with other people. (edited)

他の人とスムーズにコミュニケーションをとることは、人の成熟です。

It's immature for people to have a lot of likes and dislikes about people. (edited)

人が人に好き嫌いをたくさん持つのは未熟です。