It'd help with those novels full of side stories without any relevant to the actual plot. This would help readers when choosing novels.
A tag that will be used on pretty much every single novel on this site? Pretty useless if you ask me.
That is like assuming everything that does not have the tag contains filler. It just makes it more confusing, I think. Good intentions, but I don't think it's implementable. There's bound to be fillers when novels reach more than a thousand chapters.
But still, it just doesn't sit well on me to assume that a novel already has fillers even before reading about it. It's like the putting the horse before the cart IMO.
Here at NUF/NU, it's better to assume that any story of significant length will have fillers until proven otherwise, that's something that everyone comes to understand after reading enough novels here (unless you strictly stick to novels with relatively low chapter counts ie under 100)