No.
I spent years worrying about this exact question and never found anywhere in Scripture that calls daydreaming a sin. God made us with imaginations. Picturing yourself flying over mountains or having coffee with a fictional character is harmless.
The content is what matters, and you already drew that line yourself. When my mind drifted toward rehearsing an argument, or replaying how someone wronged me while I imagined putting them in their place, that crossed into what Jesus warned about in Matthew 5. I could feel the difference afterward, a residue the mountain daydreams never left.
What helped me was noticing where my thoughts went when I let go of the wheel. Can God Hear Your Thoughts? Philippians 4:8 became my filter for whatever is true or admirable. Flying over mountains passes that test easily.