Is a comma necessary in "What’s funny, is …"?

Solution 1:

No, it would not be better.

The reason is that it's a single clause, with "What's funny" as the subject of the verb ("is") and "Cat's dry humour" as the complement.

Never separate the subject from its verb with a comma.

Solution 2:

A comma isn't necessary there.

"What's funny" is a noun clause. It's similar to, for instance, "what you need."

A loan is what you need.

So:

Cat's dry humor is what's funny.

It's the same thing inverted:

What's funny is Cat's dry humor.

Solution 3:

It was standard for much of Modern English history to use a comma to separate a complex subject (“What’s funny…”) from the rest of the sentence (“…is Cat’s dry humour”). This is in line with the purpose of punctuation in general—to indicate prosody. Many speakers actually do pause at that point in a sentence, so a comma seems only natural.

In the passages below, [,] denotes a comma which is present in the original text, but is widely (albeit arbitrarily) considered incorrect in contemporary English.

Treason against the United States[,] shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort.

A well regulated Militia[,] being necessary to the security of a free State, the right of the people to keep and bear Arms[,] shall not be infringed.

In short: it’s best avoided unless you’re intentionally writing in the style of the early 1800s.

Solution 4:

The New Oxford Guide to Writing By Thomas S. Kane {1988} doesn't accept the mantra 'Never separate the subject from its verb with a comma' as being an inviolable edict:

The main elements of a sentence – the subject, verb and object – are not separated by commas except under unusual conditions [bolding mine]. Very occasionally when the subject is not a single word but a long construction, such as a [lengthy] noun clause, a comma may be put in at its end to signal the verb [ie make the reading easier]:

What makes the generation of the '60s different, is that it is largely inner-directed and uncontrolled by adult-doyens. [Time Magazine]

In such a sentence the comma between the subject and the verb may help readers to follow the grammar.