I've launched a few businesses and projects and worked in startups for years so hopefully my insight is somewhat valuable. :)
This sounds like a good idea, but I think there are two questions you need to answer:
1. Is there any competition?
2. If not, why is no-one else doing this already?
If this isn't being done already it's probably because it's very difficult as this definitely seems like a good idea. If I understand correctly you're attempting to build a marketplace. I'm in the process of building a marketplace and have worked in a marketplace startup in the past. I can tell you it's extremely difficult because you have to market to both the supply side (advertisers) and the demand side (app developers).
Targeted Ads: if you're going to be bringing in advertisers to show their ads in mobile apps you're going to have to take into consideration the fact that each app may have a very different audience. That means you will need to customize which ads are displayed in which apps, which introduces a whole extra layer of complexity to your system.
Ease of Use: This needs to be extremely easy for the advertiser, customer and developer. If it's not easy for any one of the three groups it will not work.
Proof of effectiveness: Both the advertiser and the developer need to know that your system works. That means building in reporting, and probably tracking conversions for both the advertiser and the developer. Not an easy task by any means...
Pricing: This is extremely important. How do you make money? Make sure your incentives align with both sides of the marketplace. The developer wants to maximize payment they receive for displaying advertisements and the advertiser wants to minimize their cost to acquire a customer. You need to make sure that the better you do at meeting their incentives the more money you make. It will encourage you to build a better system and reward you for giving your customers what they need.
Last, but certainly not least, evaluate and see if there is reasonable way you can do this leaning on existing advertising networks. If you can minimize the work on the supply side (getting advertisers) it will greatly increase your chance of succeeding. Once you have a strong network of developers you can always go out and recruit your own advertisers.
If you want to chat more let me know and we can schedule a call. :)