As was said earlier, Flemeth looks forward while Solas looks back, which ironically makes him a lot like the Dalish in many ways? I think that not spending the intervening millennia in Uthenera has given Mythal a perspective that Solas sorely lacks, he thinks only of the now and of acting when he can, whereas she thinks ahead and the ramifications of what acting will do?
That's probably why she's had a far better relationship with the Dalish than Solas has, because I think she was lot more careful with how she approached them, realising that they were a proud people and did not like to be preached at or made to feel small?
It might be unfair to say, but the whole "What's up nerds, you're all wrong" gag is probably not too far from what likely happened when he tried to reach out to them, where his genuine attempts to offer them knowledge and history were rebuffed because he didn't respect them enough not to preach at them or make them feel foolish for what they didn't know?
We can see that clearly in how hurt Lavellan was when Solas reveals the truth of the vallaslin to her? Even as delicately and carefully as he tried to break the news, he still caused her to be greatly upset by this revelation and discovering that her people had gotten it so wrong? It's not too far-fetched to imagine that wouldn't have been so careful whilst telling a regular Dalish, who he lacked that level of connection with that news? Being a little heavy-handed with his info-dumps is probably why they react to badly to him?
I think that's why Flemeth chose to become Asha'Bellanar to the Dalish rather than reveal that she was Mythal... she didn't want them to worship her and follow her instructions out of a sense of blind obedience? Instead she choses to help them indirectly, giving them just enough hints and advice so they can solve whatever problem they have by themselves, which helps them to grow as a people and become more independent?
(Incidentally, I think that perhaps Morrigan mistook Flemeth trying to instill the same kind of lessons in her to be "bad parenting", rather than the help that it was intended to be? Not to say that Flemeth might not have been the best mother to her, but considering that Morrigan is a intelligent, independent and exceptionally driven woman whenever she sees something she wants or has a problem in front of her that needs solving, it seems Flemeth's lessons did take root within her?)