The long on top, shaved on the sides cut? It's accurate - here's an example of it in an advertisement from 1916, and here's one from 1911 - but a little too prevalent in the show. Short back and sides, slicked down on top was the most common haircut for men around 1920. There are a lot of photographs that look a bit borderline, but I generally think that the sides in these cases look too shiny to be shaved rather than pomaded.