Apparently Lemmy copied the new reddit layout which shoves everything into the middle of the screen and wastes a massive amount of space. Even on the monitor I oriented vertically this is the case: the post I'm typing right now is using like 30% of the available screen real-estate and wasting the other 2/3rds.
My philosophy has always been that if reddit removed support for the old style, that's when I'd stop using reddit. Switching to Lemmy is like switching to new reddit though.
I made an account, but I can't really see using this as a replacement. I'd guess (but I might be wrong) that the type of people clinging to the old reddit style are also the most likely to do something like switch to Lemmy out of principle.
(I looked around and it doesn't seem like there are any browser addons or userscripts to restyle it either.)
Just delete the container-lg class off the first div you see it on in the app and make a userstyle.
There are hacks that can make it somewhat better, but people usually only put the effort into doing that kind of stuff when they're already committed to using something. So that's not really going to encourage people to switch.
This is what I came up with Stylebot but it doesn't make the experience good, it just makes it somewhat less bad.