Depends on the content of the DLC and whether I think I'm getting my money's worth out if it.
I only bought the Sword/Shield DLC because I wanted the fancy party dresses that it unlocked. Playing through the DLC game scenarios was simply something I did out of obligation. Whether it was worth it or not is still up for debate, but my trainer character does look fashionable, so I did get something out of it.
Cosmetic DLC is only a consideration when I like the outfits included. 99% of the time, it's a no.
I did buy most of the DLC for Fire Emblem: Three Houses; not so much for Engage, since its extra story scenarios didn't seem like something I'd want to play through.
I'm all in for buying the state map DLC for American Truck Simulator, however. Each one gives me hundreds (if not thousands) of new roads to explore and the dev team goes to great effort to make each one as accurate as possible, even sending staff to tour each state beforehand as research. Plus I need to support them if I have any hope of them making it all the way to the East Coast, lol.