It depends what you mean by "nice".
I care deeply about my friends (very carefully chosen!) and family and will do anything for them, even to my own detriment. I am extremely loyal and always try to support those people in whatever way I can.
I don't suffer fools gladly or have any patience of idiocy or rudeness or lack of self-awareness. So can be quite abrupt if people behave in ways that I find unacceptable. Zero tolerance for selfish or entitled behaviour or people who won't take responsibility for themselves and the things they do. Or people who cannot be bothered to learn about things and then complain when their deliberate ignorance backfires on them. No sympathy from me for that.
I see this as a positive because I have learned boundaries finally, and will defend other people being treated badly too, but I can imagine that other people do not like it when called out or blocked for being useless or nasty. I show everyone respect until they demonstrate that they don't deserve it, and then they will get none from me. I have limited energy and won't expend it on people who are not worthy of it. Is that not "nice"?
I love my children and do absolutely everything for them as their lone parent, financially, emotionally and physically. But sometimes I'm ill and tired and irritable so they probably think I'm not be "nice". And I feel very guilty for that.
I suppose there are two levels to it: the deeper side of you and whether you have good intentions for others, or are selfish and will only help people if it is of minimal inconvenience to you or actually benefits you. On that measurement I am definitely nice.
The shallow level that acquaintances might see - particularly if they behave in a repulsive or unpleasant way - then no I will not be "nice" to them. And will not care one bit if they think I am "nice" or not.
I think the worst people are the entitled people who are just out for what they can get from others they know and from society in general, take take take, with no thought for the impact on anybody else. Yet you might meet someone like that and they could behave as if they are empathetic and thoughtful. Because they know this will benefit them, to put on this pretence. I don't think such people are "nice".