Yes, there is sometimes a trade-off between truth and optimal signaling; and in those cases, if you aren't willing to lie or good at lying, rationality makes your signaling worse. But not always; there are far more cases where it's just a matter of recognizing what you're signaling and how, and fixing any incorrect signals. In those cases, rationality makes your signaling better. I believe the effects from the second case are usually stronger, so that becoming more rational represents a net gain, albeit a smaller gain than if everyone loved interacting with honest truth-seekers.