“What are your career goals?” (also asked as “where do you see yourself in 5 years?”) is a common interview question. The aim is to find out how you might fit in to the employer in the longer term, as well as to ascertain what kind of person you are.
Mumbling “I haven’t really thought about it” is probably not a good answer. It suggests that you have not thought beyond the salary, and additionally that you are a drifter who is not in charge of their life. On the other hand “I want your job” will come across as aggressive, and could imply that you will engineer their departure.
Instead, say something like “I want to learn new skills and be an asset to the company by meeting and exceeding performance targets. I hope that this will enable me to progress within the organisation.” This shows ambition, but also an awareness that you will have to perform to be promoted.
One instance where this may not be so appropriate is for a candidate for a job for which they are overqualified, and when age may be an issue. For someone five years from retirement, perhaps emphasising the wish to do that particular job well may be better.