First of all, when does any respectable Janna ever get AP? Not that building AP items is wrong on Janna, but usually a person who knows how to play Janna doesn't even touch AP. On the flip side, though, Karma needs to build AP to be effective. Her Heavenly Wave, Spirit Bond, and Soul Shield all get better with AP. Not to mention, her passive is just that: AP, depending on how low her health is.
Karma is more of an AP nuker, a la support. The only buff she actually needs is a change to her Spirit Bond. The Mantra only increases the movement speed/slow to double the effect. It doesn't increase damage, it doesn't stun, nor increase or lower stats.
You're only comparing the ratios. What about the base shield, mana cost, duration and cooldown?
Yep, not really that much different. Karma vs Janna for shields; cooldown is 2 seconds longer, it costs 10 more mana, it has +40 more to the base affect, and they both last 5 seconds. Then again, when Mantra'd, it's 100% of the shield absorb.