The Mig 21 and the Mirage III have been criticized for lack of range, is this a limitation of the wing configuration?
No. Their range limitation stems from their original concept: relatively cheap simple lightweight agile fighter (MiG-21) or interceptor (Mirage III).

There are plenty of long(er)-range aircraft with delta wing (if that's what you mean). Concorde, for one. B-58. Eurofighter.

For a given class of aircraft, range is primarily a function of fuel quantity and therefore (almost directly) of the maximum takeoff weight. JF-17 is 35% heavier and so has significantly greater range. (It also helps that it and its engine are two generations newer). J-7 is practically the same as MiG-21 and has very similar performance.