IIRC, she came with a couple hundred mana crystals along with her B/G fat pack when she was first released. That may have been what drove up the price, since Ajani and Sarkhan both just came with runes and a fat pack.
The short answer is what you wrote but not as much as one could think.
Ajani has huge strength in both his 2 skills(not counting the third here) and insane mana gains.
This mana gain coupled with the insane efficiency(power/mana ratio) of white AND red creatures give him a big boost.
When you couple it with efficient removal, there is little the opponent can do.
To really shine kiora needs specific cards from both colors that pull off sinergy/combo decks.
When you have it the net power skyrockets but if we talk about quick battle leaderboard she also has a big downside: effect animations.
Those cards slow down your games while animation play for buffs/card draw/card fetch.
Ajani is not as much affected.
TLDR: kiora is stronger with proper cards but not that much faster in winning matches.