Mircoblog Theme, Disable Login

Hello, there.
It is indeed possible to do what you are asking, and it is pretty simple.
You need to sign on to your admin panel and, on the left menu, unscroll the “Settings” section and click on “Discussion.”
In that window, under “Other Comment Settings,” you need to uncheck the option “Users must be registered and logged in to comment”.
Voilà! The job is done.
If you are going to have open comments, I do suggest you ask your users to fill in their names and e-mail addresses to prevent spam. I also suggest adding a captcha to your comment form: there are a lot of really simple plugins you can use for this (one of them is actually called “Really Simple Captcha”). Another must in my opinion is to enable the Akismet plugin that is included in the core WP installation.
Hope this helps. Cheers!