In 1889, American journalist and humorist Edgar Nye introduced the phrase "A mile wide and an inch deep." He was referring to a river found in the Midwestern and western United States, called the Platte River. The Platte is a muddy, wide, shallow, meandering stream with a swampy bottom, these characteristics made it too difficult to ever be used as a major navigation route. Though the Platte is an important tributary system in the Missouri River Watershed, it was disqualified from use because of its lack of depth.
Nye wrote that the river "had a very large circulation, but very little influence. It covers a good deal of ground, but it is not deep. In some places it is a mile wide and three-quarters of an inch deep."
And so the phrase was born. It's not meant to be a compliment. In fact, it quickly began to be used in politics, academia, and other fields to describe people whose knowledge is superficial.