There's no harm in leaving out the mushrooms, as far as I know.  I never put mushrooms in my soup, others leave out the celery, others leave out the peppers.  It's a personal thing, I think.  I would just make the soup however it will taste best to you.  After all, if you don't like it, you aren't going to want to eat as much as you should, and that will defeat the whole purpose of the diet!

Good luck to you!

i always leave the mushrooms out. Disgusting. I love my soup and add lots of fresh herbs and spice to mine. Yum yum but i never leave out the celery or the peppers. In fact for some extra spice i add a half of jalepeno to my soup when I am in the mood. Yum but wear gloves to seed them it will burn under your fingernails or if you touch your eye by accident. Washing hands doesnt help much!!