Most Helpful Girl
I think it was low and immature for her to end the relationship via text. If you expressed that and she feels bad, well, she should. Given her age though keep in mind she probably did that way because she was too embarrassed and didn't want to face you. She didn't want to see the hurt she could cause. I don't think you should feel bad because you let her know how it feels and hopefully she'll not be so callous to do that in the future to another guy.
Most Helpful Guy
No, I think you were right for pointing that out. Relationships should not be ended over the phone like that. The same thing happened to me once, and it felt even more hurtful just because it was done that way (we ended up getting back together though). It felt impersonal. She not only lacked the courage to tell me in person, but she lacked the respect as well. Your now ex-girlfriend disrespected you by ending your relationship over the phone like that. I would have said the same thing if I was in your shoes (and I once was).