Tricky. And one that I know is going to be a problem for me too, as correcting printed material is my trade. I don't see very much connection between "what your DD's teacher has to deal with on a daily basis" and not being able to spell, but I'm not in a teacher's shoes - and if a teacher becomes conscious that you're likely to correct her comments, she may, as I think larks35 was trying to say, just write "well done" and leave it at that in order to avoid your pink pens. This honestly would not be helpful for your DD as she'll miss out on valuable feedback.
I think the way I'm going to deal with this is that I'm going to tell my DS if his teacher has made a spelling (grammar, punctuation etc.) mistake in his book, point out what it is and explain what would have been correct. But I'm also going to be very clear with him that people DO make spelling mistakes, that we DON'T need to tell her about it, and that it DOESN'T mean what she's said is wrong. That way, hopefully, I can simultaneously keep up standards in his English, not undermine his teacher, and get him to understand that spelling mistakes do not necessarily mean that someone's wrong, stupid, or ill-educated. It just means they've made a mistake.
I do understand the indecision around how to tackle it, OP! So hard to find a way that satisfies both the demands of education and tact!