Did or did not is past simple. Do or do not is present simple. "They are very straightforward if they don't like the way you treat them. This sentence is more grammatically correct. If they do not like the way you are treating them, as in now, then they will be honest and tell you. To make the first sentence grammatically correct, it would need to be something like this: "They would have been very straightforward if they did not like the way you treated them" But this sentence would mean something different to the other one. It would mean that the event has already happened, and that if they were unhappy about the way you were treating them, they would have let you know about it. Did / did not = past Do / do not = present

"They were very straightforward if they did not like the way you treat them". "They are very straightforward if they don't like the way you treat them. Tenses must match each other, present with present, past with past.