There are “bad games” and there are very bad games. In Jackson’s last four starts he has had two very bad games. The open question is are the two bad games an anomaly or they results of defenses “solving” for Jackson’s highly unusual skill set.
To me the closest comparison to Jackson’s skills in QB history is Michael Vick.
Vick had a pretty successful NFL career. I expect Jackson may have similar success. If you consider Vick to have been a great QB then you can certainly put Jackson in that level.
If defenses have “solved” Jackson and he and Roman cannot counter the solution, then Jackson’s results will likely trend in an unfavorable direction. Time will tell.