Albeit there is a excellent answer, wherever a stat is good or not is very much relative to what competition it faces.
The best example of this is the Speed stat. Salamances speed of 100 is great compared to Tyranitars 61. But if you instead compare Salamance with Garchomp who have 102 Speed, Salamance is relatively speaking not so good (and trust me, those two extra speed can be absolutely crucial).
And this is really the case of every stat. Gardevoir is almost never used in higher competative batteling because it's stats is incredibly similar to Alakazams, and it's just to relatively weak to properly function. That being said, it's stat still aren't so terrible it's never used. In the certain division it's actually used Gardevoir is a powerhouse, since it's stronger than its competition.
So there is no set value of a stat being good or bad.
97 Speed in OU is below avarage and isn't that good because of the myriad of faster Pokemon.
97 Speed in UU is quite fantastic since almost the entire tier is slower than 97.
With all this being said, some extreme stats are, relatively speaking or not, are really really good. Blisseys HP or Ninjasks Speed is really really good, no matter how you look at it. But this is only in extreme cases. Generally stats are relative to the most common oponenent, and it's only in extreme cases the can be properly solidified as good or bad.
I know this is a very fuzzy answer, but I hope it helped.
