I haven't heard of bogus ethernet adapters causing issues unless we're talking some no-name brand from Alibaba. There's a good chance you bought the same one as me from Amazon which hasn't given me problems: https://www.amazon.com/gp/product/B00MYT481C/ref=ppx_yo_dt_b_search_asin_image?ie=UTF8&psc=1
What I suggest is to try out your other Switch games to test out their connection. Or you can setup a Battle Arena with competitive ruleset to attract someone who probably has a wired connection - or ask a friend/Discord rando to join your arena and test the lag. If it works, then it's the opponent's bad connection, but if it doesn't, it's something to do with yours.
This is the one I use on my Wii and WiiU
https://www.amazon.com/UGREEN-Ethernet-Adapter-Nintendo-Chromebook/dp/B00MYT481C/
This is what I have on both my Wii and WiiU
https://www.amazon.com/UGREEN-Ethernet-Nintendo-Chromebook-Raspberry/dp/B00MYT481C