If you stay in academia, you will often end up in situations where you have the option to do nothing. (Usually, they do not look like this one; usually, they will involve you going home early because nobody checks if you are in your office/lab working on research.) You will not succeed if you always take the option to do nothing. One of the ways that staying silent about this situation may reflect poorly on you is that other people will conclude that you'll approach research the same way.
There are ways to take advantage of this situation that are not dishonest. If the professor isn't taking initiative to come up with things for you to do as the TA, then you can take that initiative yourself. (For example, if there is a recitation section or problem session, you can ask if you can pick your own topics to cover that are relevant to the material discussed in class.) However,
- don't do this without the professor's knowledge, and
- don't do this if you don't think you can do a good job.
With these two caveats in mind, it might be worth your while; you get to do more interesting things, you learn more from being a TA, and the students get more out of the class than they would if the TA did nothing.
Whether you do this or not, you need to talk to the professor (and to the department, if talking to the professor proves impossible).