Is a Thai bank passbook sufficient as proof of funds when entering Thailand?

Nope, that won't work.

There are two distinctly different times when people can be asked for proof of funds.

You can be asked for proof of funds at the thai consulate when you apply for a visa. At that time you show a bank account (thai or foreign) for the required amount. You CANNOT show cash.

You can be asked for proof of funds at an entry point into thailand when you're stamping in. At that time you show cash (thai or foreign) for the required amount. You CANNOT show banked money.

Has to be in cash for border crossings