ASP.NET includes ASP.NET Web API for creating rich REST-ful Web Services that return JSON, XML, or any kind of content the web supports! ASP.NET Web APIs can provide data services to mobile apps like Windows Phones, iPhones, Android and more. ASP.NET Web APIs can be used in any ASP.NET Web Application, including ASP.NET MVC, Web Forms, or Web Pages – it's all One ASP.NET.
ASP.NET can also power mobile applications with responsive design frameworks like Twitter Bootstrap included out of the box with VS2013. You can use any CSS framework or open grid system you prefer, or perhaps select a powerful mobile system like jQuery Mobile or Sencha and great mobile applications with PhoneGap. The choice is yours, it's all possible with ASP.NET.