Do you think it's acceptable for me to ask the girl sitting next to me on the bus to stop talking on the phone
Why do you want her to stop?
I'm sick of hearing her talk about how she thinks her previous exes are losers which ones are great in bed
It's normally good to respect other people's privacy and just move away to let them continue their phone conversations.  However, if the bus is crowded, and this woman is being loud and inconsiderate, then you can ask her to quiet down for the sake of others.
I'm going to go ahead and ask her
You're doing the right thing by letting her know she's being disruptive to other people on the bus. She might not realize how rude she's being.