Postively, in this case, means "absolutely" or "certainly true." It's not postive/negative.
You look positively beautiful! You look really, really nice.
You are positively filthy - into the bath with you, young man!
The author means that Basil Fawlty became an archtype used to describe other people. You can say "Oh, that's so Basil-like" to mean "that was something Basil would have done."
(That show was positively hysterical!)