Well, since personality was explained, Imma talk about looks.
Looks really depend on how much you care about them. Since I do a lot, being a pony drawer myself, I take the most time in this department. However, if you don't draw then it would be much harder to take time in this, no?
I, personally, don't like the pony creators. Simply because they are quite linear and each OC becomes a copy of other OCs. And if you want to stand out, it would be much better to get a requested image (These can be found in Creative Commons). I could probably make a quick one for you too, if you give me a brief description and.... time.
Now the cutie mark on the other hand is a bit more annoying. You have to have a cutie mark that fits in to the backstory. Of course, it should match the name quite well, and shouldn't blend in with the pony. And it should, hopefully, catch the eye.
On another note for backstory and personality, what a lot of people don't get is that if your parents are murdered, you should be a bit more insane then just angry. Also please don't use that cliche.
Hopefully that helped and wasn't too much rambling!