The cloud's outline seems a bit jagged, you migth want to smooth that out, the character seems plain, you need a more...dynamic pose for a mock up, the terrain seems simple and empty, you migth want to add an enemy, more ups and downs to the map, maybe some water, trees, shrubs etc. the dirt seems like it never ends which isnt very good unless your making an 8-bit styled mock up.

 Good mockup, but, lacked details.

Not bad, but I think you should make the heart a little brighter so it stands out more. Also, you have outlined the clouds in light blue, which creates a blurring effect. Lastly, you could add more to the level. But I absolutely love the stule used here, and this style definitely has potential.