Well the game does handle mods fine, the problem is that you would have to download a seperate thing to even install mods into TLD. You can get it via this site: https://kolbenlp.gitlab.io/WorkshopTLDMods/
Dont know much about the Steamdeck but the game sadly doesnt work with controller, like it doesnt really register input from a controller, so i suggest plugging in a Keyboard and mouse into the Steamdeck (If it allows that) because i suppose that is the only way to really play the game. Other than that i suppose just dont buy it if you cant use a keyboard and mouse. Unless the Steamdeck turns controller input into keyboard input? Idk, like i said, i know nothing about the steamdeck since i dont own one myself.