I agree that she shouldn't have become a working royal straight away.
She wasn't British, and had no experience of British life, let alone royal life. She'd only been in the country for a few months! And it was always likely, given her age, that she'd be looking to start a family soon, and indeed she went on to have a baby within a year of marriage.
Maybe the Queen should have insisted that she take a few years to settle into royal life, get to know the country and the role, and of course focus on her baby, before taking on royal duties? She could have attended events unofficially and got to know what was expected of her without taking on a formal role herself. After all, it was expected to be a lifetime role, so why rush things? But instead she got major Royal patronages, big international tours and high profile engagements, right off the bat.
That was an obvious mistake in retrospect, but surely the Queen or her advisors must have been aware of the potential risks at the time, even if they had no clue just how unsuitable she would turn out to be?