Where is this scene originate from in Episode 5 of nichijou? What is this thing that Mio's cloud refers to? 

And what does this text on the screen mean (The text on the screen, it's like an old Japanese font-style) ?


  "15:40  1-Q in front of the podium"


Based from the word "sunk", is that like boat or something ? 



Relevant video on Youtube, check at 0:39.
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.