Mary's diary page is a paper note in the Fallout 76 add-on Wild Appalachia.
Location
The note is found in a backpack inside a semi-submerged trailer on the northwestern edge of Clarksburg. It is obtained during Unsolved: Picnic Panic.
Transcript
I'm done crying. Chelsea used to be my best friend. Now she keeps saying all kinds of mean things. I know I'm smarter than her, but she's prettier than me. Why can't she be happy about that? Why does she have to be the best at everything? Just because she's a Wesley and the Wesleys are so rich doesn't mean she's better than me.
Chelsea wants to go on a picnic and play some games. She got so mad last time when I beat her. She cheats all the time and it makes me so mad, but I don't say anything. Dad says I have mom's temper and I need to control it. I promised him I would try real hard to not get angry if she cheats or calls me names.