Yes, the actual stock should be slanted.
If you take it off, the receiver extension should be slanted as well. I believe Mossberg uses UTG receiver extensions.
If you don't like the slanted buttstock, replace it with a Magpul MOE, the standard model. They're only $39.99 msrp now, Magpul lowered the price to accommodate for the SL line.
It's a cheap improvement.
By standard model do you mean commercial? So replacing it with this (MOE® CARBINE - Magpul® Original Equipment - MOE® Carbine Stock ? Commercial-Spec Model) would eliminate the slanted feeling?