Ahoge (アホ毛, アホげ), literally “foolish hair,” is a visual cue common to Japanese anime and manga. Consisting of a single cute, often long, lock of hair sticking out from the top of the head, it is most often used to identify foolish, bumbling or carefree characters.
For those without context…
Source
That’s not exclusive to anime, Alfalfa here was originally created as part of “Our Gang” back in the 30s:
and don’t forget “Our Gang”