Where is this scene originate from in Episode 5 of nichijou?
In this scene Mio is being referred to as a boat that exploded, or was hit by a bomb (of embarrassment :P) and sunk. The image intended as a visual pun based off the combination of:
a mushroom cloud (from explosion) + Mio's head/signature blue hair

The text on the screen furthers this as it refers to the time and location Mio was sunk in a similar way that one might talk about a ships time and location when it was sunk.