It should not. Any slice/artboard marked for export should be extracted as an SVG, which is the medium that Animator uses to render the assets.
That said, if you want to rasterize assets in Sketch, you can -- use Sketch's Layer => Flatten Selection to Bitmap, and as long as the bitmap is marked for export, it will be treated as a bitmap in Animator. This is useful esp. in cases where performance is bottlenecked on complex vector assets.