I don't think that really tells us anything. Fitzgerald got a lot of his redzone looks because he's so good in the redzone, often when he was heavily covered. I watched a lot of SD games this year (own Rivers/Gates in my dynasty), and it seemed Vjax got redzone looks only when he was open.
Obviously one guy is going to have higher success rates when he's getting the throws only when he's open, as compared to a guy who's getting the ball forced into him at times even when he's covered.
Where do you get your redzone stats? I'd be interested to see if some of the player's with the highest success rates in the redzone aren't mediocre or below average receivers because of this.
For instance, I'd bet Calvin Johnson's redzone success rates are among the worst, because they forced it to him in heavy coverage very often there this year.
I get all the redzone data from the Data Dominator. Fitzgerald's First Down% in the Red Zone this year was 43.5%. Here's a brief list of players who averaged at least 1 red zone target per game who topped that figure-
Andre Johnson
Tony Gonzalez
Marques Colston
Sidney Rice
Roddy White
Heath Miller
Visanthe Shiancoe
That's 5 of the best receivers in the league, along with a pair of TEs who have based their entire careers on being a weapon in the red zone. Not really any "mediocre or below average" receivers there- not even any second options (provided you count Rice and Shiancoe as options 1A and 1B in the red zone). You're right that VJax had fewer red zone targets (he just missed my 1 target per game cutoff)... but among non-qualifiers, only Vernon Davis and Antonio Gates could match his efficiency. Gates, on the same number of targets, had 2 fewer receptions, 1 fewer first down, and the same number of TDs. We already know that Antonio Gates is one of the greatest red zone options in the entire NFL, and he's on the same team as VJax catching passes from the same QB, but VJax still marginally outperformed him.
Anyway, the sample sizes involved are all very small here, which is why I used a 2-year window in my last comparison. On that 2-year list, VJax ranks 29th in targets, but 12th in first downs and 12th in TDs.
I understand that Jackson's usage level in the red zone isn't anywhere near Fitzgerald's, but I do believe that he is potentially every bit the red zone weapon that Fitzgerald is.