This question is a bit misleading, initially, because often the words “nice” and “kind” are used synonymously, although the two words are distinctive. “Nice” has to do with being pleasant and agreeable—even in a hypocritical sense. “Kind” has to do with being benevolent, considerate, and humane genuinely.
So, are Christians commanded to be nice? We are commanded to be kind and loving. In fact, kindness and love are two of the fruits of the Spirit (see Gal. 5:22-23). Nowhere in the Scriptures, however, are we commanded to be nice.
This might come as a surprise to some. Please understand, we are not to be mean spirited in any way. However, there are times it is okay to not be agreeable. For whatever reason, in American Christendom many have come to believe that we must never upset or offend others. Yet, when we observe Jesus in the Gospels, He never watered down His message, He was willing to speak truth although it sometimes upset people, and He was willing to let people walk away. Mind you, He was always respectful and He recognized the value of persons. He spoke the truth in love. Nevertheless, He called a spade a spade, and He refused to bend the truth to accommodate anyone or their sins.
Jesus was not striving to be nice when He told Nicodemus, a religious leader, that he must be born again. He was not being nice when He accused the Pharisees of being white-washed tombs, as He declared their hypocrisy. And He was not being nice when He allowed a number of people to stop following Him because they did not like His hard message of eating His flesh and drinking His blood.
Jesus loved each of these people. He valued each of them enough to give His life for them. Nevertheless, He never compromised the Gospel. He never condoned sin. He never budged an inch just to keep a person from walking away from Him.
So, are we commanded to be nice? No. We are to speak the truth in love (see Eph. 4:15), but we are to stand for truth even if others are offended. Certainly we are to try to keep the peace, but never at the expense of truth.