I'm going by watching his play. He doesnt make good reads at all and often makes poor decisions. His fallback is to run and that's not gonna end up well. Perhaps he'll turn the corner but hes had a couple years with no change in his play, so doesnt look good. They will probably give him a season or 2 more because of where they drafted him, but after next year if he doesn't have any improvement, they'll be drafting another guy. Jay Cutler played better for them than mitch does. Mitch just has a superior defense, Cutler did not.
And wonderlic means nothing as far as being able to effectively play QB. Guys with poor skills excel often and peak scores almost never mean great QB play. Steve young is one of the only exceptions I can think of.