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JAY NAILED THIS ANSWER RIGHT ON THE HEAD !Not necessarily but if it's getting a lot of natural traffic then it's definitely a good sign that it might be ranking well.
If a site is getting tonnes of traffic from natural sources then it means that the site is probably getting shared a lot on social networks and has a lot of people linking to the site both of which help your site to rank well.
Of course if you just have a one page site that isn't optimized for the search engines at all and has no backlinks or anything but is getting a tonne of traffic from say Facebook ads or some other paid traffic source then your website is still going to rank poorly.