Kimetsu has more online popularity in Japan than Black Clover, whether you’re using the amount of twitter followers the official series account has, the number of tweets it gets each week compared to it, the number of fanart it gets compared to it, or the number of likes and retweets on any tweet on Kimetsu’s official account gets compared to BC. It’s plain obvious.
It’s mostly because Kimetsu’s audience is different to BC’s (Kimetsu having an older one than BC, which we’ve heard is mainly made up of kids) and more passionate about it.
As of actual manga sales popularity, BC is bigger (but comparing them now is dumb when one has an airing anime and the other doesn’t)