Am I rude for being quiet?

No.  As a customer I prefer you just do your job.

Well if a customer asks you for assistance with something then you should help them as a staff member. Other than that, as long as you serve the customer fairly then you're under no obligation to do anything else

Not exactly, however if a customer speaks to you it would be polite to respond or at least acknowledge the customer.

Well, if you're making zero social interaction that is often perceived as rude. 

Small talk isnt necessary for just baggers..but at least a small gesture of acknowledgement or a simple thank you goes a long way. People like to be noticed. 

Being quiet doesn't make you rude as such. But, it can make it seem as if you are boring or disinterested. Depending on whether you also have resting ***** face, it can make you appear surly.

Surely, you can manage a smile or a good morning

nah you're just doing your job. people are annoying so don't stress (:

No your job doesn't require you to talk.

It's more of the cashier's job to make small talk anyway.   I think it's fine