Both sentences are understandable, meaning the celebrities are trying to increase awareness about diabetes.
One aims to do something, and one aims at an objective
Your first sentence might be better as
...were aiming to create awareness... ( where there was none before )
...were aiming to increase awareness... ( to a greater level )
Your second sentence is a bit verbose in construction, possibly better might be
The celebrities that organized the marathon were aiming at increasing awareness about diabetes treatment.
creating an awareness
implies there are several different kinds or types of awareness, whereas with awareness there is usually differing levels of understanding.