Well yeah, on the whole. Liberalism, especially in it's modern, progressive form, put's forward a kind of philosophical and moral framework whereby individuals respect each others actions, their rights and freedoms and live to tolerate ideas, faiths and values they may disagree with. A basic definition of nice would be: 'that which is seen to give others pleasure', and the moral framework of liberalism rests upon that basic idea, that others should be respected and given the same rights and freedoms as everyone else. Contrast this to, for example, an ethno-nationalist, and you can see how they don't fit the definition in the same way liberals do. Ethno-nationalism is by definition exclusionary, seeking to exclude others by virtue of ethnicity; thus they aren't being 'nice' to certain groups of people. It's a bit of a stupid debate, but yeah I think Liberalism is an inherently 'nice' ideology.
Even if you simply meant 'well-mannered' or 'pleasant', I would say Liberals are more likely to fit this definition than a Conservative, a Fascist or a Marxist. Those groups are either radical in opinion, and thus seek to voice their views in a way that would not be considered 'well-mannered' or 'pleasant', or are intolerant of certain people, ideas or ways of thinking.