In my games, being nice does not mean that a character is good. Lots of villains are nice to people they like (and to people they want to deceive). Perhaps more to the point, a number of neutrals are prevented from being good by the desire to be nice, to avoid conflict, and to avoid confronting unpleasant truths. To paraphrase Dumbledore from a recent movie, they do the easy thing rather than the right thing. So, being nice doesn't mean that a character is good.
Whether there is a correlation between being good and being nice is another matter. Kindness is a virtue. Cruelty is a vice. A character who is actively and intentionally cruel could easily become neutral. So, I don't think that a good character can be horrribly mean. That said, being assertive, abrasive and even confrontational is not always the same thing as being mean. It would not be inappropriate to cast a good character in the role of Patton slapping the soldier who he saw as a coward. It's not deliberately cruel, nor is it necessarily mean (though the papers back home will portray it that way and get him removed from command). However, it's definitely not hand-holding at a tea party and it's not "nice."
In a recent campaign, I played a paladin and probably the biggest conflict in the party was between him and the party's cleric (same alignment and the same deity). Why was there conflict? Because my paladin was an aggressive, assertive, and decisive take charge kind of character and the cleric didn't have any of those characteristics by nature... and the cleric was supposed to be in charge. If something needed to be done or something needed to be said, my paladin was the kind of guy who would do it or say it. The cleric felt like that was undermining his authority.  Combined with our different assessments of one of the NPCs' motives, this turned out to be what was probably the most heated and persistent interparty conflict in any campaign in the two years I've been playing with the group. But both characters were good aligned--and appropriately so.