Quick thoughts. This is not really a theory or anything, but the sewed mouth of the human in the clown outfit made me think of Eto and how she sewed Karren`s mouth and eyes. At the moment I think Eto has not part in this Clown incident(I can`t really think of any reason why she would…), but to make sure I decided to compare the way how the mouths were sewed.
Here is the human showed in the recent chapter:
It seem to be that one stitch per a point was used that goes vertically up in different places of the mouth. Now if we compare this to how Eto did Karren`s mouth and eyes, it is very different:
The way Eto sewed Karren eyes and mouth was that she created these little X markings, also known as cross stitches.
Eto probably has nothing to do with this, and I think by comparing the way the mouths were sewed it shows that at least she was not the one who did this, since it very different that Eto`s previously shown way/style. Since Uta has been making mask for a quite long time, I think he would be familiar how to do something like this too. So maybe it was him who sewed the mouths? Or maybe even Kanou?