Some possible thoughts and responses on such feedback:
- Do you find this behavior to be disruptive in the classroom?
- Does this behavior violate any school rules or create safety issues?
- Does my child's "seriousness" result in social problems?
- Does my child seem unhappy or isolated?
If none of these are an issue, then be aware that some children simply prefer non-fiction to fiction, enjoy what is "real" more than what is "make-believe", find more entertainment in what is educational over what is silly, and are very observant allowing them to quickly go into problem-solving mode about things which may stymie age-mates. Classmates may look to such a child as a leader, sometimes causing teachers to take exception (while others may see the child's behavior positively, as levelheadedness and maturity).
Some kids like "Spongebob Squarepants" and others may be more interested in learning about real life forms under the sea from National Geographic, etc.
I've known families who insisted their child believe in Santa, the tooth fairy, etc through 6th grade "to preserve their childhood" and I've known children who figured out at a very young age that these were mythical characters and yet could accept that many enjoyed them... One child with a double grade acceleration felt it was important to update the 3rd grade teacher, therefore wrote a note to the 3rd grade teacher saying that if this child mentioned these characters, it was only to play along with other classmates who still believed!
Vive la différence!