Why Mitsuki take off her glasses when reading on her phone?
presbyopia in her age?
Maybe she's just far sighted and finds them annoying when reading? It's a bit odd though.
She is probably near sighted (myopic). That is what the "near" means, you see just fine nearby, you only need glasses. It is not odd at all if that is the case. I am myopic and so is most of my family, we usually take our glasses to read, it is actually a bit uncomfortable with them. My mother was very advanced in her presbyopia before she started needing glasses even at short distances.
(also, removing glasses to read is the opposite of what you would expect of someone with presbyopia)