Is the Apple TV power cord worth upgrading? HDMI, Ethernet?

No, but people that read my posts know that I am against changing power cords. I have never noticed any difference between power cords - and I am not tone deaf! Remember that sound/video starts with a component's power supply - not with the  wire connecting it to an AC outlet!

For the other cables Monoprice is fine and low cost! Don't be taken by the claims of the golden ear moneyphiles! IMO the whole cable industry is just out to profit from the gullible with no objective proof of claimed superiority!

Higher quality HDMI cables do make a difference. Not sure you need to spend thousands. With regard to the power supply, maybe, but I have never tried, nor intend to. I run two Apple TV’s. Regarding PC’s in general, I 100% disagree with the above, but YMMV.