There is a difference between being a good person and a nice person. This doesn’t mean that you can’t display both attributes, but make sure that you always prioritize being good over being nice.
A good person is willing to tell others the “hard truths.” They are willing to say no to something that they know the other person wants if they feel that it is not in that person’s best interests. A good person does not indulge in an action (or inaction) that may cause harm to another individual.
A nice person is polite and can be described as a “people pleaser.” A nice person might hold back telling someone information that, while hurtful, may help them in the long run. A nice person may omit truths or tell stretched versions of the truth (“white lies”) to spare someone from temporary pain. A nice person wants to avoid conflict, even if that conflict is necessary.
An example of being good versus being nice is that a good person will cut their friends off from drinking too much, whereas a nice person will “give a drunk a drink” because they don’t want to tell them no.
Again, you can be and should try to be both a good person AND a nice person in most situations. But when you are in a position where you have to choose between being good or being nice, always choose to be good. It may rub people the wrong way, but it is for their own benefit that you say or do what is right, not just tell them what they want to hear.