OK, here is what I found after wasting a half hour or so on research:
The picture supposedly originated on (her?) Instagram, then spread like wildfire on Reddit and other celebrity sites. It is *universally* identified as JoJo, with many, many hits. That's no guarantee, but usually, if a pic is fake/wp, you do a reverse image search on it and get only one or a few results in odd places, different names on the pic, etc. That's not what happened here. This pic is *all over* the web, with 40 Google hits. Another (NFS) pic from the same set has 124 hits and shows the *front* of her, but cuts off just above the chin, so you can't really see her face. See here:
And if you want to try to compare chin, hair, boobs, belly buttons & butts, you might try this or other fairly recent bikini pics of JoJo:
I don't do social media at all, so if one of you SM experts can try to run this to ground on Instagram, it'd really help prove me right or wrong, but at this point, I'm about 67% of the opinion it's a legit pic.
@MrHate is usually right about these things, but I'm not sure about this one.