No, it isn't. People like to point to the simplistic or even occasionally silly sounding dialogue, but it really has no bearing on the quality of the story telling or the film's overall reception. The story itself is reasonable, contains no obvious plot holes or motivation problems, and traces it's pedigree to Kurosawa.
We can pick nits and try to re-write history, but you're talking about a film that was nominated for Best Picture, Best Director and Best Original Screenplay (Annie Hall won those instead). It did win 6 Emmy's, Golden Globes and Grammys. Critics loved the film and Ebert praised its "pure narrative". So while Star Wars is the contrarian SF fan's favorite punching bag, nearly everyone at the time treated it as GREAT filmmaking, including the story.
I'm a serious genre fan who has never heard of Planet of the Vampires, so I'm not sure it is an example of successful genre filmmaking. But reading about it doesn't sound like it fits with a comparison to low budget superhero or SF shows but is more of almost an art film with limited distribution. The fact is that comic book fans have had many, many opportunities to make serious TV shows like Captain America successful, but they largely were not. The Incredible Hulk did well, but by heavily adapting the character to make the story about Banner and illuminate all other super-powered characters to focus on a much more grounded story more closely resembling The Fugitive than Superman.
There are many shows that are successful because of their genre-based fans, like Buffy. But there is still a kind of celebration of camp implicit in that and other Joss Whedon shows, which is why they are viewed as "cult classic" instead of straight genre. But making a completely straight laced genre show virtually requires a non-genre fan base because the genre people are not all going to like it, and there aren't enough of us. As a genre fan, I am more critical of comic or SF films than most people - and I am not alone in that. SF and comic films and TV really need to get some accolades before I bother to go see how much the director bent my favorite fiction out of shape. Some of it has been great, like the Nolan Batman films, and some of it has been merely watchable due to good effects and actor charisma, like Iron Man - both of which had enormous budgets and were designed for broad appeal.
Media that appeals to genre lovers only appears to rarely be good for anyone, including those fans.