The inboard mic on the ipad is not a calibrated measurement mic, so there are going to be deviations. It is fine to use to "get an idea" of your room RTA, but it should not be used to do serious room calibrations. Google is your friend here:
Scroll down to the "Conclusions/Recommendations" section. I think it comes down to you get what you pay for. The $17 Dayton iMM-6 seems okay, but the $75 UMIK-1 USB mic looks better. The RTA software on ipad may not perform realistically -- the tests showed deviations from measurement using REW on a laptop, especially under 90hz (which is probably where you most want to measure).
Personally, I use an Audix TM-1 measurement mic with REW on a laptop for my stuff.