How are you to listening to the mp3 after it is bounced?
is it in another app like iTunes? if so, make sure there is no EQ there being applied to the audio. Are you listening within Logic? If so, is it on a "clean" track with no plug-ins on it, and routed through a clean signal path with no bus/aux plug-ins influencing the sound.
And are you monitoring the mp3 afterwards through the same physical outputs as you are when you are working within Logic?