Yes, the resolution of the snow heightmap is the same as the landscape’s resolution. Each point of the landscape = a point that can be used for your weight maps. (Assuming you don’t resize, or rebuild the layer’s .raw files at a different resolution, which would be silly.)
I’m not sure what resolution you’re using in World Machine, but you can try increasing the landscape’s Normal Build Resolution. If you’re building a 505x505 landscape, then you might want to look at a 1009 or 2017 build resolution, (or any of Unreal’s recommended landscape sizes) as it will give you more detail. If you need to keep the same game-scale, then you can just scale the landscape down, so you have a denser mesh for your landscape in the same play area. All of my landscapes in the screen shots are 2017x2017 resolution, but I’m thinking I might double the resolution and reduce the in-game scale by 50% to give me a denser landscape mesh. (By default, landscapes tend to come in with a XYZ Scale of 100.)
If you’re not using one of Unreal’s recommended landscape sizes then you might want to change your resolution to something close to that, then give it another go. Results tend to get weird when not using one of the recommended sizes, which could explain your results.