It's a cute monster and a cute store. You'd do better to have a premium shop and your designs on a wide range of products, rather than each design being limited to one specific product.
My only suggestion is that the monster could have a few more details drawn onto him / her to look more professional - e.g put some light / shadow onto the head to look more three dimensional.
Some of my designs are a bit simple too and this precise thing is on my 'to do' list.
Can I ask what wordpress theme you have used for your blog? I like it.