What is it to you?
The definition of "filler" is exactly that. Something that fills in the gap in a story.
This doesn't have anything to do with canonicity though. The only form of canon that exists is franchise canon, which means that every official work under the Naruto name is canon. The only criteria that is used officially is whether or not the work is part of the continuity. As long as any official puts the work into the continuity, it is part of the story.
The only canon that exists is franchise canon, so any official work in the series is canon. I'm not sure how the definition got so distorted.so would you say something like the ostrich ninja is canon?