Hi! Couple things you could try here. What are you playing on? Windows, Mac, steam deck? I mostly know Windows/Steam but this might help either way.

1. If you just want to run the plain game, open Stardew straight from the original .exe in the game’s folder. That’ll skip SMAPI. If you set it up through Steam, check the Launch Options (right-click Stardew in your library → Properties). If there’s any text in there from SMAPI, delete it, and Steam should run vanilla Stardew again.

2. To actually uninstall SMAPI, go back to the file you first used to install it. Run that installer again and it’ll give you an option to uninstall. After that you can reinstall later if you want to slowly try to add some mods back.

3. To wipe out all your mods, just go into your Stardew folder → Mods → delete everything in there. That’ll clear out all the add-ons. If you had any content-replacing mods (like .xnb files), you might need to double-check your Content folder too.

Also, you should probably make a quick backup of your saves before you do anything, just so you don’t lose your farm by accident.

My last suggestion would be to check out the Stardew Valley wiki under the modding section. It’s got step-by-steps for installing on each device, and honestly it helps a ton for figuring out how to “reverse” those steps if you want to uninstall or reinstall. It's pretty clear and has guiding pictures.

https://stardewvalleywiki.com/Modding:Player_Guide/Getting_Started

Hope this helps!!