A dime a dozen is a phrase that means whatever you're talking about is abundant. Which is to say there is a lot of it. So if I say "cars like that are a dime a dozen" it just means it's not particularily original.

In this case it implies that the friends are worthless, cheap.

It means cheap quality and has a very negative connotation. It's an insult. I remember when it was real money and it's a beautiful coin. Smaller than a Nickel because it's silver. You could eat dinner for a dime. We have many idioms with it. Stop on a dime. Drop a dime on someone. It's your dime. Five and dime. A dime bag. March of dimes. Spare a dime. Dime store. Fun Q !

Extremely common, plentiful.