I can understand that teachers don't like being judged by people who aren't teachers, but your role is a public one. You are seen by support staff, volunteers, parents during one to ones and the children themselves. You are, like any industry, staffed with people who are incredibly professional, skilled and perfect for teaching. You also have people who do it for a living and do the bare minimum and you have the folks who've done it for years and lost the enthusiasm for the job. Not all long serving teachers are like that, but some definitely are.
My daughter, last year, had the worst year ever. Her teacher was a long serving member of staff and she was crap. That's not just my opinion as a parent...I worked in the school and knew her reputation before my daughter joined that class from TAs who had worked in her class and struggled to make sense of her agenda, from my own time in her class and finally, this year, I've heard my dd's new teacher saying how wonderful her class is and how brilliant the kids are.
The teacher in question moaned and moaned about what a crap class they were in the staffroom and she said that she had 'retreated' from them, as they weren't bothered. The whole class. Not just the kids who were behaviourally challenging, but the whole bloody class. Including my motivated, committed, focussed little girl. As you can tell, the woman is completely off my Xmas card list!
Complain about this member of staff if it's affecting your child. Even if it's based on hearsay and what your daughter is saying to you, you can't risk that the whole year will go by without anything being said, hoping it'll sort itself out. We did that last year and our girl is now behind her colleagues in the other class and is having to do extra homework and extra classes to catch up.
If enough people complain the head will do something about it, even if it's just to retrain this person and help them through any difficulties they might have. It might even make this person realise that they are no longer right for the job. Unfortunately, playground gossip isn't always wrong...and if it's backed up by your own childs unhappiness, or discomfort, you should act. As someone said, teachers can't be dismissed for being rubbish (which is appalling in my opinion) so the worst that will happen is that the situation will be investigated properly.