I would say, "What's obvious is that they do not know each other very well."

it's obvious that they don't know each other very well
or:
they obviously don't know each other very well

...depending on the context

Die Formulierung Obvious is... ist auf jeden Fall falsch. Don't use it it you can help it :-)

To explain the difference between "what's obvious is that..." and "it's obvious that":

The normal way to say it would be "it's obvious that" or "obviously, they don't know..."

"What's obvious is that" would be used to set off this idea against another idea. ("Obviously they're in a hurry", with response "What's obvious is that they ...".)

A bit hard to explain, now that I try. Better to use "it's obvious that" or "obviously".

I agree with all the above, but would add: be a bit careful about using the word "obvious". It may be offensive (suggesting that it's not worth saying but for someone's foolishness). Germany "offenbar" is sometimes more tactfully translated "evident(ly)".

Obviously you can't start a sentence "Obvious is..."