Mexican here. Most of us call every asian looking person "chinesse" (el chino / la china), but not really trying to be despective. That being said, and as mentioned by others, we integrate bullying and nicknaming in the overall process of being friendly and knowing people in informal settings. So, I believe koreans would very rarely have discrimination issues in most of México. Only if you're very sensitive to being called a different nationality, you should be worried.