It feels so incomplete without it! What's Galaxy 1 without 2??
Honestly, Galaxy 2 was even deemed by the developers to be a Galaxy 1.5 or, as we would say now, "DLC before they were a thing on the Wii". Galaxy is the groundbreaking game that expanded the Mario universe, added Rosalina and all the gravity-based mechanics, saying that Galaxy is incomplete without 2 is being even more aggressive than the Nintendo marketing team at the time of Galaxy 2 (gotta love the DVD showing how to play).
Ultimately, Galaxy 2 was the first game from the director of Super Mario 3D World so in the grand scheme of things it led to Plessie, Draggadon and Wingo, plus the discovery that the linear levels with abstract settings worked a lot better in Captain Toad Treasure Tracker while Super Mario 3D World worked immensely better in an open world environment.
Plus, Galaxy 2 had an amazing soundtrack.
But if you ignore 3D Land in the collection, it's fine to ignore Galaxy 2 too, in my opinion. It was a good game, but basically a glorified DLC, even according to the developers. And I say that as a

fan. And as someone who actually started with Galaxy 2, then jumped to Galaxy to have more context and eventually returned to Galaxy 2. Both Galaxies 100%ed too.
EDIT: oh, yeah, I had forgotten about Pom Pom. Add her as one of the worthwhile consequences of Galaxy 2. The Hisstocrats, Cat Toad and the victory march of SM3DW too, if you are lucky enough to even remember at least 2 of those 3 without having to play the Switch port.