check if the user is owner of current page

Per the Codex, is_author() checks to see if you’re on an Author Archive page (ie, a page listing all the posts/pages/etc that a given user has authored). If you’re on any other type of page, is_author() will return false.

What you’re looking for sounds more along these lines:

if ( in_the_loop() )
{
    get_currentuserinfo();
    print ( get_the_author() === $GLOBALS['current_user']->display_name )
        ? 'It is my site.'
        : 'It is not my site.';
}

This code must be used in The Loop.

References

• is_author()
• get_the_author()
• get_currentuserinfo()
• in_the_loop()