Can I use Ajani Goldmane, once hit by Stigma lasher?

[Carnac]

Though I wont gain life, can I still use the +1 ability on Ajani Goldmane, at least to increase his loyalty?

[griffin]

From the Stigma Lasher part of the Eventide FAQ:
-- Spells and abilities that would normally cause that player to gain life still resolve, but the life-gain part simply has no effect.

So yes you can.