Can I run top as screensaver?
If you're looking for hackerish screensavers, the old Xscreensaver bundle was converted to OSX a while ago.

Nicely hacker type examples would be Sonar, AppleII, and Phosphor. Possible GLMatrix too.

However, to answer your question: https://superuser.com/questions/420546/osx-run-bash-or-app-as-the-screensaver

Write an applescript to run Terminal, run Top, and fullscreen itself. Then put that script into the app.