The Interestings has a pretty straightforward title. It very clearly refers to the tongue-in-cheek name the group from Spirit-in-the-Woods gives themselves, even if they only use it for a limited time. So the title, then, is a shout-out to this group of people we meet and follow from their teen years into adulthood.
On another level, though, the title references Jules's relationship to the group specifically. They may sarcastically dub themselves the Interesting as teens, but for Jules, this group of people—and particularly Ash—become more or less a lifelong obsession. Jules is relentlessly fascinated with Ash and her family and money, downing bottles of wine in envy as an adult. And because of this, Jules fails to really become her own person—she only ever understands herself in comparison—which ultimately makes Jules a less interesting person than she might be otherwise.