Yes, they can, provided they are used as a source-identifying image for the manufacturer or retailer.  So what happens if the emoticon is commonly used by others for non-branding purposes?  They can still be used as proprietary marks by a business.  The U.S. Trademark Office has registered several popular emoticons for use as marks with a variety of products and services:

😉

:o)

🙂

*-)-

🙁