I mentioned this elsewhere, but don’t forget back in the day you were probably on a display where 800x600 looked totally fine and so when the expansion came out and gave us that option, it looked so much better than launch Diablo 2 pre-expansion. Now, of course, you are (I assume) on a 1080p, 1440p, or even a 4K display and when you toggle over it’s not like the game auto-sizes to a nice 800x600 window, it’s probably full screen and all the jaggies.
I played this last probably 5 years ago in an 800x600 window, which was a PITA to set up iirc, but it looked pretty darn good - when played in a window.