My experience with this is: I bought a Lan adapter to try to make the online less laggy, specially in games with 3 or 4 players. My Nintendo Switch is just at the side of my router (30 cm of distance). I bought the Ugreen Lan Adapter. It solved nothing, and I feel that the connection is worse, cause when I use the lan adapter, I have more lagged matches than before. I can play the game properly, in most of the matches of 1v1 using wireless connection or the lan adapter. But I feel that with wireless connection, I have less lagged matches even in 1v1.
My internet connection is great in any online PC game that I play a lot (Overwatch, Rainbow Six, Counter Strike, Modern Warfare...).
So, I was disappointed with the results of playing with the lan adapter, it helped nothing and I usually leave it disconected when playing Smash.
It may help people with Switches that are far from their router or have more oscilation on the connection, but not for me.