First of all, it’s not out of character for Kim to break up with Tommy over another man. Why? Because Kim is not a liar, she would never tell Tommy it was because of someone else if it wasn’t. It would cause unnecessary hurt to someone she cares for. What would be out of character would be for her to say it was because of another man if there were no other man because that would be flat out lying. It’s not to ‘make it final’ for Tommy, because a break up is a break up and growing apart is as final a reason as there being someone else. (Hell, even a “I don’t like the way you smell anymore” is a good enough reason as any. People break up for much less.)
If you choose to believe that Kim would go through this whole charade of writing a break-up letter because she wants the best for Tommy, if you choose to believe that point, then you’re not giving Tommy enough credit as a man, a boyfriend and as the leader of Earth’s Sole Line of Defence Against Alien Forces.
While we are on that tack, we can also flip it around. Let’s explore this from Kim’s perspective shall we? Kim must have obviously felt that Tommy was lacking something, otherwise why would she even send that letter in the first place? So either way you look at it:
1. Tommy must have been neglecting her enough for her to feel that she needed to break up with him (for his own good)
2. Tommy can’t juggle having a relationship and be the leader of the power rangers at the same time (then Zordon made a grave error in choosing him to replace Jason)
3. Kimberly definitely felt they were growing apart or a big enough change in their relationship (brotherly feelings, anyone?) to even contemplate writing a letter.
4. There really was someone else.
I don’t think I even need to address the absurdity of it not being possible that she would choose Jason.