I´m not even sure Bieber would be particularily terrible in a movie like this, for all we know he could be an amazing actor just as likely as he could be completely awful. Since we haven´t seen him in any major roles it´s kinda hard to predict, like, who would have thought that Justin Timberlake should turn out to be a good and likeable actor? Or how about Zac Effron, a guy who was mostly known for being in those awful teen muscials but who actually did a very decent job in Orson Welles and Me.
I don´t have an opinion on Biebers acting skills (as much as i hate his music), until he´s done some actual acting.