TLDR

Yes AP affects Lulu's ultimate. The more AP she has, the more HP she gives to champions when she ults.

Full explanation

Lulu's ultimate increases a champions size and total HP. The champion also knocks everyone around it up when she ults. AP increases spell damage but it also affects Lulu's ult. Specifically, 1 AP = 1% increase in the amount of health Lulu gives to a champion.