This is Michael Jackson we're talking about. After Thriller, and before 1993, he could release just about anything and have a commanding presence on the charts. If there was a single artist who defined what "pop music" was in his era, that artist was name Michael Jackson. Therefore, by definition, Jackson will still be a pop artist. That doesn't mean his music can't sound different, but his musical/cultural role after Thriller is such that he can't become a "rock singer" in the sense of not being a pop singer at all. In that sense, he can sing rock songs, but he cannot be a "rock artist"
But yes, had he been so inclined, he could have made a very rock sounding album, he clearly had the ability to write such songs. The big problem is that he desperately wanted Bad to sell, and for it to sell better than Thriller, which of course was all but impossible. That meant he wasn't really all that open to experimentation like that on a grand scale at the time. Bad is a formula record, basically it's the Blueprint from Thriller with adjustments.
The two big problems here have to do with Jackson himself. He lacked the inclination to make a largely rock album, and he was too obsessed with chart success to have taken a gamble that was so likely to fail from a charts perspective by Thriller standards.