Just as it is a bad idea to run a website for Adsense purposes and make it about an unpopular niche, the reverse can be similarly true. Now, there is an aspect of balance to be considered here. Certainly, you want to make your website attractive, and having a popular topic for it is important. No-one should try to tell you “that’s been done, try something else”. But there is more to it than that.
If you are looking to “exploit” a niche, consider exactly how popular the niche is. If it’s a fundamentally “limited” niche, then it will not bring in much traffic. On the other hand, if it is a spectacularly popular niche, there is the chance that your site will get lost in the churn of so many sites about the same subject, and you’ll lose out to more expensive, more established sites.
This makes for a difficult decision. How popular is “too popular”? That’s a value judgement and it varies from niche to niche. But if there are already twenty well established sites on the subject you want to write about, you’re going to have the mother of all battles on your hands just to get noticed. On the other hand, looking for something no-one has already done doesn’t leave you much space.
What you can look to do is bring a unique slant to the subject, such as geographically localizing it. There are lots of sites about, for example, herbal medicine. What you might consider doing is taking a local approach by talking about herbal medicine in a specific locality. Equally, you might take a more nuanced approach by making it about the pros and the cons of herbal medicine, where many sites talk all about the pros.