Quote Originally Posted by pushvv View Post
Help to understand the song.
First, I want to say that The Mamas and The Papas are one of my all-time favorite groups, и я обожаю Cass Elliot -- она была богиня, и всё. So it made me very happy to see that they have some fans in Russia!

A few comments about your specific questions:

John and Mitchy were gettin' kind of itchy
Just to leave the folk music behind; (John and Mitchy didn't want to leave folk music? Were they uncomfortable to leave folk music?)
No, the opposite -- "to be itchy to do something" means that you are very eager and impatient.

And after every number they'd pass the hat. (what is "pass the hat" - to gather money from people?)
Yes, exactly -- "to pass the hat" means "to request voluntary donations of money."

When Denny met Cass he gave her love bumps; (love bumps?)
"Love bumps" can be interpreted as (1) a euphemism for a sexually transmitted disease like herpes or genital warts; or (2) more innocently, госиная кожа от любви. I don't know which sense is meant here (but most likely it's the second meaning).

Called John and Zal and that was the Mugwumps. (mugwumps?)
"Mugwumps" was the name of their group.

McGuinn and McGuire just a-catchin' fire in L.A., (cathcing fire?)
Here, "to catch fire" means "to become popular and successful."