If you look in your project ‘Config’ directory you should have a ‘DefaultDeviceProfiles.ini’ which is how you set up your different device profiles. If you don’t have one, you can just create one and put it in there. The jaggedness is caused by ‘MobileContentScaleFactor’ which scales down the rendering resolution. It is likely set to 1 (i.e. non-retina) by default. Using full resolution for newer devices is actually a ‘MobileContentScaleFactor’ of 2.
If you wanted to increase the resolution for an iPhone 5S you would add a line like this:
[iPhone5s DeviceProfile]
+CVars=r.MobileContentScaleFactor=2