Nonlinear causality is not a paradox. If event A causes event A to happen, it's just circular causality. Perfectly logically sound, but just try to twist your brain around how it happens. If event A causes event A to NOT happen, then THAT is a paradox. Depending on how you interpret time travel, this could simply be impossible, cause the universe to explode, cause all affected people to be fated to die, or cause any number of other effects. Write your own outcome. >:3
if Past Applebloom went into the future to meet her future self, how can she have a cutie mark before she turned her age without a cutie mark.
that would imply that at some point Applebloom will loose her cutie mark and forget how she got it only to be reminded by her past self, after that the "future Applebloom will travel to the past only to come back to remind the "futureer" Applebloom how she got her cutie mark.
This is what's known as a stable time loop. The events enclosed within are responsible for causing each other, and there is no identifiable initial incident that caused it to come into being in the first place. They are anomalies of space-time, existing apparently independently of the surrounding time stream, undeterred by the known laws of cause and effect, yet existing all the same.
Or as the Doctor would say: "Wibbly-wobbly, timey-wimey".