Did I make a mistake in being too honest in the PhD interview?
I want to make two points here, both are indirect answers to your question, but still open to interpretation with regard to your specific interview.

Interviews are less like taking an exam, and more like going on a date. It is possible to do everything right, and still not click. Equally, it is possible that you do many things wrong, but the other person is willing to take a chance on you, because they see something they like and willing to invest on. It is therefore not really worth it mulling over the details of the date in any meaningful sense, since you'll never really know what it was that did or didn't click. Having said that, just like a date, you would want to sound honest, but not arrogant, and generally not put the other person in an uncomfortable position for the sake of 'honesty' if you expect to establish some sort of relationship with them going forward.

There is always a way to rephrase a negative into a positive, and it typically pays off to do so. You come across as a more approachable, less negative person, who sees the good and the opportunity in things, as opposed to the negatives. It's generally a good way to go through life, too. So, in your case, you could have rephrased

&quot;it was boring and it was the prof's fault but I'll look at it if I really have to&quot;

into

&quot;I am reasonably comfortable with the topic; however, I felt it was not given the time it deserved in uni, and my exposure to it so far has been more theoretical. Therefore I look forward to seeing an interesting application of this field in a real-life setting, and to improving my skills in this area: I'm very thankful for the training opportunities on offer in this job -- this is in fact one of the reasons that led me to apply here (blah blah, continue on positive spin)&quot;.



Regarding the second point, as a personal anecdote, I was taught this by my own PhD supervisor during my PhD. A large part of my PhD involved improving a field in which one of the most influential papers had many (in my opinion) naïve assumptions which weakened its conclusions. I approached this in my work from the point of view of &quot;we build Y which doesn't suffer from the errors in X, who did bad things x,y,z&quot;. My supervisor thought this was unnecessarily harsh and had me rephrase it as &quot;X pushed the field forward by proposing X. We improved on this idea by addressing improvements in areas x,y,z, which we believe make the general direction proposed by X even stronger&quot;.
Now, &quot;technically&quot;, both versions say exactly the same thing, and both are 'honest'. However, imagine I wanted to invite Prof X to be my external examiner in my PhD viva at the end of my PhD. Which of the two formulations do you think would predispose them more positively towards my thesis?