I am a huge film buff but I am not too familiar with film animation. With film it is hard to determine who is truly a pioneer of certain things. In most cases it is the most popular who are considered the pioneers but with film a lot of stuff gets lost to the dust so it has a hard thing to determine.
I believe Lotte Reiniger is a pioneer is a true legend and pioneer of film animations. She started her work as one of the earliest to do so even some ten years before beloved Walt Disney. It is because of the success of Walt Disney that she perhaps gets overshadowed and not enough credit or her work.
Lotte Reiniger continues to inspire animators and artists, including Google doodle creator Olivia Huynh who built almost everything, from the puppets to the top down camera rig. She pioneered a style of animation that relied on thousands of photos of paper cut-out silhouettes arranged to tell a story, before Walt Disney .
I believe the Lotte Reiniger is an important part of the history of film animation, but not the true pioneer. No one did more for film animation than Walt Disney, so he is the true pioneer. Disney's movies and animation truly changed the world, and they have a major effect on animation and filmmaking in general today.