Which Is Correct, "Fundraiser Coordinator" Or "Fundraising Coordinator"?

Both are equally acceptable, but there are just slight differences in their meaning that you may want to consider (actually maybe too subtle to make much difference at all for your T-shirt purposes):

Fundraiser coordinator  implies one who coordinates the event/events.
Fundraising coordinator implies one who may coordinate many efforts and processes to raise funds, even beyond coordinating actual events.

Beyond that,  "fundraising coordinator" would be personally preferable to me only because of how it literally sounds when articulated;  fundraiser and coordinator have the same word ending sound.