"For reference, the following Pokémon take 120 base damage from Grass Knot. From Generation 5 onwards, heavy Pokémon kick up dust as they land switching in." Quote from the grass knot page. Does this dust do anything, or is it purely aesthetics?
The dust serves no purpose other than to indicate the Pokemon is especially heavy. Worth noting that non-grounded Pokemon don't generate this dust, even if they weigh enough.