There are two important quotes to remember-
First from Paton
Wars may be fought with weapons, but they are won by men
and from Napoleon
I’d rather have lucky generals than good ones.
Any armed plane is capable of shooting down another aircraft under the right conditions.
In the Vietnam war, there were A-1 Skyraiders
that shot down MiG-17s
there was even one shot down by an OV-1 Mohawk
the JF-17 is a better plane than a MiG-21 so the Fishbed would be at a disadvantage in that dog fight, this is the first I have heard of a Su-30 being shot down -so this is either battlefield rumor or some new development. I’m am thinking this would be an issue of instantaneous turn rate vs better avionics.
It is important to remember that all that a couple of kills prove is that either the pilots were better due to training or they were luckier.
You can test airframes maneuverability with an algorithm
Then there are issues of avionics but at the end of the day it all comes down to pilots in a cockpit being able to make the best of the situation.