I think that Pinterest is a great place to market your app since the majority of pinners use smartphones (at least according to my analytics).
Just do what your competitors or other app makers are doing. Most likely it's creating a profile, making boards and groupboards that your target audience might like, ask to join established groupboards in your niche, follow users in your niche, etc. Lately, I've been seeing more and more promotions via private messages. And if you have a budget, go for promoted pins.