There are much better nonogram clients on the App Store, sadly. The clunky interface has difficult to read numbers and some odd grid-filling behavior. For instance, if you mark a cell with an "X" and then run over it with your finger while filling in cells, it will invert the "X" rather than leaving it intact, which makes it rather easy to introduce errors, especially when this behavior is combined with the auto-fill feature. And with both permanent on-screen ads AND unskippable interstitial ads, it is not worth putting up with these hassles to play the game.
It's a shame, since the idea of stitching together smaller nonograms to make a larger image is a nice touch. I just wish the implementation were friendlier to the player.