What is '"Wunderbar!" he exclaimed with great relish' a reference to?

Literally, it means ' "Wonderful!", he exclaimed with great enjoyment. '

Wunderbar is the German word for Wonderful. But, to English speaking people, it's a silly word. It's funny just to say, "Wunderbar!". Carrey is adding commentary to the end of it, to make it even funnier. He's not actually referencing anything, just a silly word followed up by a 3rd person commentary for added effect.

As already said "Wunderbar" is a german word which sounds silly to some non-germans. The addition is breaking the 4th wall to the audience, because he is stating bluntly what his acting is supposed to convey. Sometimes instructions like that are written in scripts.

It would be interesting to see the script of the movie, maybe the line was actually a script instruction that Carrey ad-libbed to his lines.

In addition to that it is also a multi-level-hyperbole, because wonderful is already quite superlative terms (unless you use them in a polite BE-way). Wunderbar is one-upping it by adding a silly sound to it. His acting is adding one more stack on the superlative. And finally, he is adding the mildy-spoken instructions to the mix.

The exaggeration adds to the comedic delivery.

It is a parody of a type of bad writing sometimes known as said-bookism, where authors keep a book of alternative words to use instead of said. Examples:

• "Yes" he muttered.
• "Yes" he whispered.
• "Yes" he roared.
• "Yes" he exclaimed.
• "Yes" he hissed.
• "Y-Y-Yes" he stuttered.

A variation is to use said, but add an adverb or other descriptor.

• "Yes" he said angrily.
• "Yes" he said with contempt.
• "Yes" he said in a low whisper.
• "Yes" he said happily.

Or, they can combine the two:

• "Yes" he hissed in a low whisper.

One or two examples in a story is acceptable. More than that and it becomes annoying. Famously, the Tom Swift series of books used them excessively. Here's a sample:

"Oh, I'm not a professor," he said quickly. "I'm a professional balloonist, parachute jumper. Give exhibitions at county fairs. Leap for life, and all that sort of thing. I guess you mean my friend. He's smart enough for a professor. Invented a lot of things. How much is the damage?"

"No professor?" cried Miss Perkman indignantly. "Why I understood from Miss Nestor that she called some one professor."

"I was referring to my friend, Mr. Swift," said Mary. "His father's a professor, anyhow, isn't he, Tom? I mean Mr. Swift!"

"I believe he has a degree, but he never uses it," was the lad's answer.

"Ha! Then I have been deceived! There is no professor present!" and the old maid drew herself up as though desirous of punishing some one. "Young ladies, for the last time, I order you to your rooms," and, with a dramatic gesture she pointed to the scuttle through which the procession had come.

"Say something, Tom — I mean Mr. Swift," appealed Mary Nestor, in a whisper, to our hero. "Can't you give some sort of a lecture? The girls are just crazy to hear about the airship, and this ogress won't let us. Say something!"

"I — I don't know what to say," stammered Tom.

So, the movie line is mocking this type of prose.

There is also a type of pun called a Tom Swifty which combines a parody of this type of prose, with a pun.

• "I arrived by parachute" Tom explained.
• "Look at my wedding ring" said Tom with abandon.
• "You must be our mysterious host" Tom guessed.
• "I'll sue you again" Tom retorted.

I thought the line might be a Tom Swifty, but I can't see a pun there.