I was taught that Mercury, Venus, Earth, and Mars were "Inner Planets," and the others were "Outer Planets," with the distinction being that the asteroid belt separates the two. I think the intention was these terms applied just to our solar system, but thinking about it, it's not wrong to refer to planets outside our solar system as Outer Planets, either. I don't think they taught us any significance behind why we have the distinction, though, so it might have just been fill-in-the-blank fodder.
Ice is a solid (okay, fine, most of the time), so if Pluto is mostly ice, then Pluto is mostly solid. Furthermore, any planet will become a liquid if you move it sufficiently close enough to its respective sun.
And now the core does matter, but only for Earth?
There's another issue that if we stop calling Jupiter, Saturn, Uranus, and Neptune planets (or plane-ettes, or wandering stars, or rudisplorks), we have another question to ask ourselves, "What are they?" I have lots of ideas, but I think the only category that is board appropriate is to call them Space Balls.