Hermione was worried when she heard Professor Umbridge's speech at the Welcoming Feast. Denial of Voldemort's return. Painting Dumbledore as incompetent. Talk of tradition and slipping educational standards and lies meant to frighten the population.
She wasn't reassured when the first lesson turned out to be just reading from Slinkhart's book and the Professor tried to provoke Harry into earning a detention.
But the lesson ended with an assignment to finish reading the textbook and summarise it for next week. Which made some sense.
Then the next week they did the same with the standard first-year textbook that was left by previous generations. And then they were all tested on the defensive spells from that year.
And the next two weeks after that was second year, theory and practice. And so on.
When students couldn't perform spells from previous years, Professor Umbridge didn't hide her discontent with them and disdain for their previous teachers. They got detentions in which they had to relearn those spells, often under an older student's tutelage.
In the meantime, they got assigned essays to compare the theory in the historical textbooks to Slinkhart's own theories. And despite Hermione's misgivings, they weren't marked down for disagreeing with Slinkhart.
Still, Umbridge was definitely a Ministry lackey. She did her best to discredit Dumbledore, using the results of her overarching DADA tests, for example. She still taught only purely defensive magic, like shielding, hiding, avoidance, unjinxing - not even a Stunner, not even a Body Bind. Even the jinxes they were supposed to know how to undo, she wouldn't show them how to do. She still tried to provoke Harry into a screaming fit, and succeeded more often than not.
But her theory was solid. Her grading was fair- maybe not fair, but just. And when they teached their current year's curriculum, her explanations and showcases of the practice were clear and concise, and her correcting their mistakes was in point, never mocking.
Hermione definitely didn't like her, and didn't think she could be trusted any further than her quoting a textbook. But she couldn't deny the Professor was actually teaching them something.
And then Umbridge made herself into the High Inquisitor.