your sprite seems to be changing height every other mockup ... is this supposed to be so?

I agree with mozzy, try to come up with something more complex. Spending a lot of time on a piece with a lot more pixel technique will benefit you a lot more in the long run then a bunch of quick little simple pieces. Perhaps you can come back to the idea of a mini mockup after you have learned how to best use the minimal amount of pixels to your advantage, as opposed to feeling like you must oversimplify due to the smaller size.

You could easily fit more detail and character into this size without losing focus or readability but you'd need a better understanding of pixel technique to do so. Good luck!

you could at least try to challenge your self