Although he's the star of the show (and the titular character—Stoker didn't name this book Harker), we really don't get a lot of intel into our main man.
Dracula is more of a shadowy presence in this novel, always threatening the main characters, but rarely making himself visible. He's usually just off-stage, just ahead of the good guys. Does that mean he's incredibly clever? Van Helsing doesn't think so: He's done some research and knows that Count Dracula was a great scholar in his own day—but that was a long time ago.
Since becoming a vampire, he's lost some of his intellectual skills and hasn't learned a whole lot. And being smart in the Middle Ages doesn't put you much ahead of the curve in 1897. Nobody told Dracula the ol' "use it or lose it" rule... at least according to Van Helsing's narrow definition of intelligence.
Seriously, though, why does Van Helsing say that Dracula has a "child-brain" (23.2-4)? Dracula has been around for centuries, after all. He can turn into a bat and a wolf, and can control the weather. He's seen a thing or two. Just because he doesn't know about fancy new technology doesn't mean he's got a "child-brain." All he ever wanted was to hang out in London, maybe catch a show, and suck the blood out of the teeming mass of people. It's such a huge city, no one would miss a few people.
Partly because we know so little about Dracula from the novel, many later filmmakers have tried to develop his character based on the few tidbits of information we do know about him. For example, when Dracula tells the vampire brides to lay off Jonathan Harker in Chapter 3, he comforts them by saying, "Yes, I too can love; you yourselves can tell it from the past. Is it not so?" (3.35).
What, exactly, is Dracula referring to? Did he, at one point, have a real romantic relationship with one or all of the vampire brides? During his life, was one of them his wife or lover? Does he have a dark history? Some kind of tragic love affair in his past? Too bad: Stoker ain't going to tell you.
The novel doesn't give us answers to these questions, but some later films try to develop them (see, for example, the 1992 film Bram Stoker's Dracula, directed by Francis Ford Coppola).