Thanos didn't know who the snap would kill off, which you could argue adds to the stupidity as he could be leaving all of the greedy wasteful beings alive. It was "fair" tho.
And let's be real, most villains have questionable ass plans if you really break them down. Killmonger, Luthor, etc. It's about how piff their journey to accomplishing their goals is. Thanos ranks at the top there.
The ones who just want to wreck shyt make the most sense - Vader/Emperor, Joker, Ocean Master. I thought Vulture in HOMECOMING was dope too, how he wanted to keep it lowkey.