Yes, and for many a reason.
It could be that the answer to the question is classified and that Mueller will be happy to answer the question behind closed doors. It could even be that merely offering this on the grounds of classified information, would give up something that needs to be protected.
Another reason is that similarly there are some elements of the report Congress is not allowed to see. I know one contention is that Grand Jury Testimony is can't be disseminated to congress as it could discuss specific charges that were not upheld by the Grand Jury as well as general information not meant for public dissemination (such as documents with information that is private to a specific individual. Bank accounts, social security numbers, ect.
In both cases, these matters can be discussed in closed door sessions much more freely and Meuller was scheduled to have a number of closed door testimonies. If he was to publicly testify, he would have committed a crime, and he does have a right to not testify to anything that could cause him to violate the law.
Finally, Mueller may have been asked his own personal opinion on a matter which is not relevant or he is not in any position to give. Mueller's personal opinion on any individual is not something he needs to respond too as he's here to discuss factual statements and not what he thinks should happen. Questions about his opinion on the President's personal character or whether or not he would recommend impeachment are political and opinionated in nature. The former does not speak to any factual finding in the report and the later is not an opinion Mueller is in a position to give as he is not a member of congress. He can only speak to, e-hem, "Just the Facts, Ma'am" and only then to the facts he found and reported. It's important to remember that Mueller is an agent of the executive power of the government and thus subject to the Hatch Act (which limits what Executive agency employees can say with respect to politics while in a duty status) and Executive Privelege which is a limitation on which deliberative communications internal to the executive branch) so while he is a servent of the government, he may be obligated to not answer certain political questions and may not be obligated to answer questions about discussing matters with his supervisors and employees when they are posed to him by congress, which is a different branch of the government that is not superior to the executive.
It would be helpful if you could give specific questions he answered in such a manner.