As you already know, nobody likes Google+. This comment is part of the campaign to protest in order to show the dislike and negative feedback of Google+ on YouTube, and ultimately, for Google to remove Google+ from YouTube altogether.
Bob is simply an icon for this campaign. He's like a mascot. A stick figure by itself doesn't really have a meaning, but after someone on YouTube sees this comment multiple times, he will associate the Bob stick figure with the anti-Google+ campaign. Therefore, next time this person sees Bob, he will know the person who posted the comment is against Google+ on YouTube. Another purpose the stick figure serves is a way to get a person's attention. Most people would stop scrolling to read a comment with ASCII art (in this case, a stick figure) over a comment with only plain text.