I don't know.
Perhaps the owner of that house likes them,
so I guess she drew such hippo on the wall.
You can come back to this town later (Episode4),
then you can also notice that a mirrar of the school's toilet was a "one‐way mirror".
In Japan, these elements which are unrelated with the game itself
are said that these are "Asobigokoro" (遊び心).
Of course I like them,too.
I make these elements at my next mod,too.