Kishi never explicitly stated in the manga that Rikudou Sennin created the tablet, however we can make an educated guess that he did.
Let's take a look at the facts.
- To read the entire Uchiha tablet, the Rinnegan is required.
- Madara is stated to be the second Sage of the Six Paths. (This should mean no one else had the Rinnegan inbetween Rikudou Sennin and Madara.)
- The tablet had an extensive list of things, like the Moon Eye plan, how to obtain Mangekyou Sharingan, how to obtain the Rinnegan, and the history of the God Tree with Kaguya.
Kaguya could not have written the tablet because most of the tablet's info pertain to Rikudou Sennin's era, where he had the Rinnegan and was the Juubi Jinchuriki. And since Kaguya isn't considered to be a Sage of Six paths, we can assume she doesn't have the Rinnegan either. We can also assume that a Rinnegan user must have written the tablet.
With that said, Rikudou and Madara are the only two candidates left, but Madara uses the tablet to learn about the history of Shinobi, so Rikudou is the only one who could have written the tablet.
EDIT: Additionally, Tobi stated the Sage of Six paths created the tablet here (bottom panels).