In Shelley's book, the monster seems to be very intelligent. However, in modern culture, Frankenstein's monster seems to be little more than a zombie with scars and bolts on his neck. How did this change in depiction come about?
There's really not a lot of mystery here. The popular depiction is based on the 1931 Universal Pictures film Frankenstein starring Boris Karloff.
Earlier film depictions of the monster are much different. The earliest surviving Frankenstein film comes from 1910 and was produced by Edison Pictures. There, the monster had no bolts and wasn't zombie-like. The monster wears some furs and robes instead.
In 1915, another silent adaptation of Frankenstein was produced called Life Without Soul. The film is lost but some stills survive and the monster again doesn't have any of the trademarks now associated with him. He's more of a wild-eyed vagrant wearing disheveled clothing (though since the film is lost, we don't know if he was zombie-like here).
It was the Boris Karloff depiction that cemented the image of the Frankenstein monster in the public's imagination. The makeup artist on the film, Jack Pierce, invented the green skin, the bolts, and the haircut. (Source.)
The makeup took about three hours to apply every day, and there were weights in Karloff's shoes to make his walk more convincing. With the whole costume on, Karloff stood almost seven feet tall, even though Karloff was naturally only about six feet tall.
The Universal Pictures costume department came up with the black suit, which was intentionally ill-fitting, particularly the sleeves which were too short to emphasize that the monster was supposed to be much larger than everybody else in the film.
Interestingly, when Universal was hard up for cash in the 1950s, they began threatening lawsuits against others who used their version of the monster without permission (Source.) They did the same with Dracula, because much of the popular depiction of Dracula also comes from a Universal film, 1930's Dracula starring Bela Lugosi. You had to license these depictions of the characters from Universal to use them.
The latter eventually resulted in a lawsuit where Lugosi sued Universal for compensation, because part of Universal's depiction of Dracula included Lugosi's portrayal. Lugosi won, or, rather, his heirs did, establishing case law that personality rights are inheritable to the estate of the personality being depicted, after the person's death. This was then codified into California's civil code.