Are Korean people nice, especially toward foreigners?
It's impossible to answer this question accurately by simply saying yes or no. People being "nice" is nothing more than a personal experience and everybody has their own experiences. There are definitely some stereotypes that many Koreans have toward foreigners, whether they are white, black, or Asians. Then again, who doesn't have stereotypes against others? Koreans are more likely to make wrongful assumptions about people from other countries than openly discriminate against them.
Many people believe Canadians are very nice people, which is a positive stereotype. (By the way, my personal experiences say it's true!) But South Korea has lower crime rate and homicide rate than Canada. (Korea also has a better safety index than Canada.) Does this mean Koreans are "nicer" than Canadians? I'd say, with Koreans, there are a lot more ignorance and annoyance than hatred and violence toward other people, if anything.
Let's say you are traveling in South Korea. You are walking down the street in Seoul and you are lost. You ask a middle-aged Korean man for help. There is a good chance that he will do everything in his power to help you. He may even take you there himself. Talk about being nice, right? Then again, he may turn around, make a disgusting throat-clearing noise, and spit on the ground. (Spitting on the street is very common in South Korea.) How would you describe that experience? Nice? Rude? I'd say this: T.I.K. (This Is Korea).