As you can see from Rowling's own illustrations for the annotated special edition of Philosopher's Stone and pen & ink sketches she did for the 1999 Children in Need show, the shape of the scar isn't presented consistently.
Later, in the first-edition book illustrations (which were hand-chosen by Rowling (but not drawn by her)) the scar is depicted as more of a jagged lightning bolt.
Later covers present his scar even more dramatically:
As to why JKR chose a scar in that particular shape, she's been quite mysterious but she did at least confirm that it's a "lightning bolt" shape and not a runic shape:
Q. What is the meaning behind Harry's lightning bolt scar?
A. There are some things I can tell you about it, and some things I can't. I wanted him to be physically marked by what he has been
through. It was an outward expression of what he has been through
inside. I gave him a scar in a prominent place so other people would
recognize him. It is almost like being the chosen one or the cursed
one, in a sense. Someone tried to kill him; that's how he got it. I
chose the lightning bolt because it was the most plausible shape for a
distinctive scar. As you know, the scar has certain powers, and it
gives Harry warnings. I can't say more than that, but there is more to
and that she largely chose it because it symbolizes power and intensity:
UKMCLive: Why did you pick a "lightning bolt" to be the symbol on Harry's forehead?
JKR Live: It had to be something that suggested the intensity of the pain that Voldemort was trying to inflict upon him but a
simple and plausible shape for a scar.