I'm probably not the best person to critique this since I do my own visual assets, but I thought it was quite neat. I liked the overall simplicity and range of possible outcomes. I was thinking about coding something like this for a roguelike to create interesting interactions, so it was very cool seeing something based off similar theories.

I only tried the "sprite"-export, since I usually use a *.png sprite sheet for my 2D work, but I was quite pleased that it exported the way I like it.

To answer your overall queries:
I thought that it was simple enough... Maybe labeling the top "body", "item" & "hair" under "presets" or "random" would help the user understand that this is just another way of manipulating the bars for the same variables, but it makes sense pretty quick, if you keep tapping.
I think that both navigating the scrolling through of animations and zooming should also have either a dropdown menu or way for going the opposite direction.

Other than that; I think I'd add a "jump" animation. I think the exported sprite sheets would work great for something like "Pixel Dungeon", but your page seems to suggest platformers, so it seems odd that there isn't a jump animation.

If I was you; I'd also give some sort of copyright license information about the exported data. It seems like a neat tool to at least create some placeholder art, but maybe people might be a bit skiddish to use it if they're not sure about the licensing. I'm assuming it's free, maybe with accreditation, but I actually can't say for sure. I know a lot of game devs avoid things if they don't see the license agreement first.

Best of luck, like I said; it's quite neat. I hope some people use it. I could really see it helping coders trying to get a 2D project started.