jsut check how frequently is posting on Poe trade section of this forum, and diablo 4 section,
and you will find answer which is more popular atleast here.
I mean, Poe is a game based entirely around trading while d4 is a game that permits trading but is really based around SSF.
It wouldn’t surprise me if someone said poe2 is more popular, it’s newer, it arguably should be.
But I don’t think looking at trading activity on a trading website is really a good indicator of much.