Tonks was rather good at potions, despite her clusminess, and never (much) minded the nasty things Snape said to her about her clumsiness - fair enough, potions classrooms can be dangerous. On the other hand, she really hated the way he picked on weaker students.

One of the reasons she never became a prefect was not just schoolgirl naughtiness, but because she talked back to teachers who she thought were being unfair.

Dumbledore had his doubts about how happy she'd be at the ministry, for that very reason.