Toaster oven makes my kitchen lights flicker
The problem isn't within the toaster, and adding a "power conditioner" wouldn't help. It's simply drawing a lot of power from the outlet, and the wiring impedance between the outlet and the distribution panel is causing the voltage at the outlet to drop a bit.

In general, the lighting circuits and the countertop appliance circuits should be on separate breakers to begin with, so that an appliance fault won't leave you in the dark with a potentially dangerous situation (hot and/or spinning objects).

You should try to work out which outlets in your kitchen are on which circuits, and operate the toaster on a separate circuit from the one your lights are plugged into.