How old are you, Mom? If you are just a few years older than this gal, then I'm not sure why you are surprised that she is immature. Young people often are. If you are older (and sorry, but you don't sound older), then you should understand that younger people are immature until they grow up some. Life experiences help make them grow up.
If the girlfriend marries into the family, you will have to live with your decision to badmouth her in the first place, because she apparently isn't going to lose any sleep with you not liking her.
Do you really think that you are doing your MIL any good by making her miserable with your "honesty"? Your MIL can't do a thing to prevent her son from marrying this girl. Is your goal to have someone else dislike her too? That's pretty unfair to your MIL. Your husband should care enough about his mother to have not been so "honest" himself.
Your MIL will need to carve out her own relationship with her DIL. Leave your opinions at home and let your MIL find her own way with this gal. Try to stop being judgmental of why your future SIL is pregnant so that the family isn't always in turmoil. That is how to show your young future SIL how to act grown up. Be an example of how a grown up young mom is supposed to act.
Adding people to the family is not always easy. If families got to decide on who would marry into them, perhaps many people would stay single for all their lives. That's not the way it is, though. The best way for families to deal with it is to accept newcomers as best as they can. I think you've pretty much sealed your relationship with her so that your BIL won't want to be around you since his wife won't put up with it. So be it. Perhaps next time you will decide to think twice before putting your honesty out there so much.
Dawn