She has all the articulation of a box of rocks, but my take on it was that she was annoyed with Nikki and Jayne slightly more than she was annoyed with the bitching. With regard to the bitching I think Imogen is happy to sit back and take it in, but to have it implied to her that what other people are saying is the truth and she just isn't able to see it for herself is very offensive and patronising.
It was nice to see her standing up for herself, but I don't think it was even slightly altruistic. She was standing up for Imogen, not Aisleyne.
Also...
The stutter stuff is interesting. I'll bow down to the more informed if I'm saying anything out of turn, but I'm not convinced her poor articulation stems from that at all. We've seen her at all levels of temper and whether she's super-relaxed or angry she can't seem to hold a sentence longer than a few words before relenting to a premature "y'know?" rather than actually explaining herself.