Cool! Looks like the boots and maybe the helmet are enchanted. Now if she would only drop the full set, undamaged, when you kill it!

In the event that a mob (zomb/skelly/pigman) spawns with armor (which is sort of rare in itself), the occurrence of any single piece of diamond armor is 200 times rarer than that of any piece of leather armor (MCWiki--Zombie Trivia). In comparison, a Spider Jockey will likely spawn just once out of 100 naturally-spawned Spiders, making the Spider Jockey much more common than a mob wearing one piece of diamond armor. So this full-diamond-clad skelly is especially rare (possibly one in tens of thousands). But I guess if one has been working at the mob farm all the live-long day...

To complete the info. In the event that a mob spawns with armor:
49% chance of the armor being gold
40% leather
9% chain
1.8% iron
0.2% diamond

(loosely related) The one thing I fear the most: