A Stradivarius is the name of a very famous violin-maker whose instruments are now priceless. There are two types of clouds - Stratocumulus and stratus clouds, so the husband was confused. That explains the 'joke' as far as what's funny about it. I have to agree with you, and it's not.

The goal in Password is to have your partner guess a word from clues that you give him. "Stradivarius" is a good clue for "violin," because "Stradivarius" is the most famous of all violin makers. If someone says "Stradivarius" you think "violin maker," just as if someone says "Steinway," you think "piano." What's funny is that her husband doesn't get it, and makes a strange mistake which we understand a split-second afterwards. Cloud types have long scientific Latin-derived names like "altocumulus" and many end in -us. And "stratus" is a main type so some of them begin with "stra-," for example "stratocumulus." The husband hears a long multisyllable word beginning with "stra-," ending in "-us," and having a rhythmic pattern a little like a cloud name. It's only funny if you, yourself, 1) know who Stradivarius was, 2) know what cloud names sound like, and 3) think the mistake is amusing. There is a whole family of jokes based on misunderstanding words, or someone trying to sound educated by using a big word and using the wrong word instead. The humor comes from first being puzzled and then suddenly understanding the speaker's mistake. "He can write with either his left hand or his right hand--he's amphibious." (The right word is "ambidextrous." "Amphibious" means "able to live on land or in water, like a frog.") "I'd like some decapitated coffee." (The right word is "decaffeinated coffee." "Decapitated" means "has had its head cut off.")