We can't really say much about MWPP, though, as what we see is diluted trhough an unreliable narrator seeing things through other people's eyes. So many people speak so highly of them and claim to be close with them, I think it's hard to say they were only nice to each other.

The trio is pretty nice to pretty much everyone, really. Hemrione doesn't get on with Luna all that much, but she's not outright mean to her. Ron and Harry are nice with the rest of Gryffindor, the Hufflepuffs we see them interacting with, and almost all of the DA.

We really only see Pansy being actually 'nice' to Draco.