S.H.
Stressing about it will only make her stress about it. I used to do something similar when I was a kid, and the more people pointed it out, the more nervous I got and did it without realizing it. lol Replace her "bad" habit with a "good" one. Maybe you both could work on singing a fun or comforting song when she starts doing that, without telling her to stop. That way, she's using her mouth but not sucking or biting her clothes. Plus, it's hard to sing with clothes in your mouth! :)
There may be something she's stressed about; so, maybe you could just talk with her about it, too. I would start out by acknowledging her liking for it (without passing judgement, no matter how much you don't like it), then, saying you don't want her to feel embarrassed or ashamed and that you love her no matter what. We all need a place of comfort, and our homes should be that place. So, hopefully, by you telling and showing her that she is safe at home, she will open up to you. And, LISTEN to her; no advice, unless she asks for it. I KNOW it's difficult, but sometimes, we just want to be listened to, not preached to or made to feel embarrassed by our feelings/actions, as we've already stressed ourselves out enough by ourselves. So, listen...and your instincts will tell you the right way to comfort.
I hope you can help her and your relationship with her stronger through this. These are small steps to handling bigger problems, but we must work together on them. That's what family is for, and you are a great mom for trying to help her. Take care!