An extension doesn't make sense for a simple DOM manipulation - I'd recommend installing Tampermonkey then finding a script like the following:

https://github.com/hallzy/remove-youtube-shorts/blob/master/...

You can do it natively inside ublock origin if you don't want to install an extra extension (often the case for a surprising number of simple extensions, actually).

I used to block it myself with my own filter, but after YouTube changed things up and broke it I've just been using someone else's filterlist and it works the same.

https://github.com/gijsdev/ublock-hide-yt-shorts