no. iphone uses .m4r files as ringtones and nothing else (unless your phone is jailbroken in which case you can make it use .mp3 and what not as well; and download them too).
there are ringtone apps in the app store (some even free) which contain a large variety of ringtones to chose from which can then be set in your phone. these get updated often, or so they advertise.
the other option is to convert your mp3s into m4r and sync with iTunes.