That's kind of a dick move, really. Depriving her brother of happiness like that.
Besides, he'd probably discover the strangeness in Gravity Falls by himself. He'd probably make his own journal, becoming Ford 2.0 in the process.
Also, the journal is kinda how he saved Mable from the Gnomes in the end. It put him on his toes. Without it, he wouldn't have comfirmation of his paranoia and wouldn't give "Norman Man" more than a passing thought about how weird he is.
Besides, thanks to the journal, Dipper knew how to deal with Bill the first time he showed up.
Yeah, Mable just basically screws herself and pretty much all of Gravity Falls doing that.