Well, he was a really good man and he was killed by those who didn't like him or understand him. There were also many instances when he was really nice. But niceness does not begin to describe Jesus.
Webster's Dictionary defines nice, among other interesting things (look it up) as "pleasing," "agreeable," "respectable," and "socially acceptable." When most people say someone is a nice person, they imply that she is easy to get along with, never demanding, and always pleasant to be around.
Now, Jesus was and demonstrated perfect love, mercy, and compassion, to a degree that our fallen human nature cannot begin to fully comprehend. That is true. But always agreeable, easy to get along with, pleasing, and socially acceptable? I don't think so.
He certainly took on the ruling religious leaders a bunch, challenging both their words and their actions. He criticized them to their faces and behind their backs. They hated him for it.
But what about every one else? He graphically challenged his hearers to deal immediately and radically with sin (Mark 9:43-48). He criticized his disciples for having little faith (Matt.8:26). He pronounced serious judgment on entire cities (Matt. 11:20-24). He appeared to reject his mother and family, a definite cultural faux pas (Matt.12:46-49). He severely rebuked Peter (Matt. 16:23). He referred to some of his disciples as an "unbelieving and perverse generation" (Matt. 17:17). He turned away the Syrophoenician woman (Mark 7:24-30). He criticized the lack of faith of all Israel (Luke 7:9). He actually states that he will bring division to families (Luke 12:49-53). He even dares to say that a lack of repentance will lead to perishing! (Luke 13:5).
Perhaps most famously, Jesus made a whip and drove the sellers out of the temple (John 2:12-16. He violated all kinds of social norms by talking to the Samaritan woman and even called her on the carpet for her immorality (John 4:16). He didn't seem to care that his hard teachings would drive people away (John 6:53-66). And on and on.
Here is my point: Jesus was perfect and sinless, fully God and fully human. Consequently, he was perfectly loving, merciful, and compassionate. That does not, however, equal to the popular conceptions some have of Jesus being "nice" or of Christianity supposedly being a "nice" religion. As Christ followers we are to be like him and demonstrate love, mercy, and compassion. That does not mean that sin should not be confronted (especially in our own lives and in the church), that truth should not be stated (yes, in love), and that sometimes the most loving, compassionate, and merciful thing to do is to confront someone. Love often means correction, admonition, and accountability. Those who read and follow the whole counsel of God know this. Those who are on the periphery of Christianity or who are outside looking it too often confuse being a follower of Jesus with being milquetoast nice. Not the same thing.
Now, having said that, believing in accountability and proclaiming the truth is no excuse for being a jerk! See Galatians 6:1-5. But, that is a different article for another time.