The only useless items in the game are Sonia's Book and some of the various items you can only get by hacking the game. Sonia's Book has no practical purpose, and its only "use" is a device to push along whatever passes for Galar's postgame.
A few items exist in the game code that can only be obtained by hacking, most notably Mega Stones and event items. Mega Evolutions aren't in the game, and the Stones can't be sold, this making them completely useless. The same applies to event items.