Frozen Wilds is definitely important to the overall story, but it's not absolutely essential. It's a really good expansion tho, and the machines from there return in Forbidden West.
Most of the mixed reviews is console war bullshyt. This is easily one of the best games that dropped all year / all generation, if not the best.
But if you don't like Zero Dawn, this game won't change your opinion about the franchise. It's basically a very refined version of the same gameplay (with a few new mechanics) with a continuation of the overall story of Aloy. They go full sci-fi too.