I just submitted 3 very distinct ice sample teams here all of have Froslass (which is not a coincidence, I consider it to be a top3 mon on the type after all). I hope seeing those teams helps you build with the mon. I believe the mon fits on any style of Ice team, whether you're running a team based on Screens, a team based on Snow, or a team that isn't based on either of those. While I don't think its anywhere near as mandatory as Kyurem or Alola-Ninetales (which is why I'm nominating it for A and not S), it is fairly splashable and deserves a lot more love than it's currently getting.
I think part of the disconnect here is that I don't believe that Screens Ice is the only--or even necessarily the best--way to build and run Ice. I'm not sure where this notion of "Almost all ice teams must be screens/hail offense" comes from, but let me be the first to tell you this is definitely false. I believe screens/snow offense is a fine way to run the type, but there are also other possible ways to run the type that Dewgong will never fit on.
Even so, I'd understand Dewgong's tier if it was the only pivoting option available, but that just isn't true. There's UTurn on some mons (Frosmoth, Articuno), and also Volt Switch on (scarf) Rotom-F. All of these options have their distinct advantages (Frosmoth gets Defog, Articuno gets Roost, Rotom-F compresses a scarfer), and while Dewgong's probably better than these options, I'm not sure it's better enough to deserve C rank while the others stay in D tier (or in Frosmoth's case, gets moved to D tier).
