I think there's a big misconception about what "mainstream" really means. To pubishers, mainstream is not a genre, it's any book from any genre that will sell to the mainstream audience, all those millions of readers who do not specialize in reading genre novels, but who will read a novel from any genre, if it doesn't carry the genre label.
Stephen King and Dean Koontz are both mainstream writers. So is Colleen McCullough. So was Normon Mailer.
Word count matters for mainstream novels only in the sense that word length matters a first novel in any genre. Get much over 100,000 words, and publishers get nervous because most first novels lose money, and the thicker the novel, the more it costs to print, so teh more money it loses when it tanks
But a horror novel, a fantasy novel, a literary novel, a western novel, or any other type of novel can become a mainstream novel. As such, mainstream novels run the gamut in word length.
It's just that first novel nearly always have to come in somewhere around 100K, else publishers get very nervous.