Okay, I guess you didn't get it then.
That is the OP. Mushoku Tensei and ARIA both use the opening theme song set over added unvoiced runtime to set the tone of a given episode before it actually starts proper. It's just a technique so rarely used that people assume the series doesn't have an OP.
As opposed to Mieruko-chan's, which cuts distinctly, but also gives us some repeated character development and tone setting in the lyrics for the beleaguered Miko, who looks impassive during the gags of the series, but is deeply tormented by her spirit sight. Set to a lighthearted song with not-so-lighthearted lyrics.
The alternative is to just use a random pop song as cross-advertising for both the vocalist/band/group and the anime, set to alternate animation of the series' future (as with Kimetsu no Yaiba or Fire Force) or even just random shit we never actually see in the series proper.