Absolutely! However, to build credibility with the Stumble Upon community, you'll want to do plenty of voting and sharing other content that you find interesting and high quality, not just your own. Stumbling and taking the time to vote on those pages is a major part of the website, so try to build up your experience on the site by voting and following other users first. You'll also learn what sort of content other users tend to find interesting and be better able to choose what content you share on the site.
A user that votes regularly on others' content and shares interesting websites and blogs other than the ones they've created will have much more credibility than one who simply joined to promote their own content.
There's even a toolbar you can install that makes sharing and voting as easy as the click of a button while you're browsing the web normally. Once you've built up a solid contribution to the community and you're ready to share your own content, just click on "Share" at the top of the screen and choose a message as well as your audience (specific, your followers, etc..)
Hope that helps, and good luck stumbling! :)