Is it reasonable to say I'm not happy at my current position at interviews?
A traditional interview technique is to reframe such questions.  Don't focus on the fact that your current job is bad - view it as an opportunity to discuss how the new job will be better and how you're a good fit for it.

Bad: "In my current job, I have to teach too much, and good research is not appreciated."

Good: "I'm looking for a job with more opportunities to pursue research, in areas where I can make the best contribution.  I have successful experience in doing X, Y, and Z, and University A is known for its excellence in this area."