Student or Learner
I read a joke on the net, following which someone commented 'Now, there is a thought.'
Is it equivalent to saying 'Good thinking'?
Yes; "now there's a thought!" suggests "that's something I had never thought of before, and which is a good or interesting or startling idea".
After a joke, it's usually humorous or ironic. Elsewhere, it's often (but not always) serious.