is it possible to be too nice?

[Zanmato]

Yes.

If you're too nice people take advantage of your niceness. It depends in what way you mean being nice. Sometimes you need to put yourself first but you can do that in a nice way.

also I don't trust people who are 'too nice' nobody is 'too nice'

This.
Many people took advantage of my kindness and since then I trust only those who are trustworthy and fully deserved this title.