Is there any difference between ‘it’ and ‘so’ as a complement of ‘she looks’?

Solution 1:

I think "she looked it" is more idiomatic.

It's equivalent to saying "She was the picture of sorrow"

Saying "she looked so" is correct, but a litle unusual.

Most readers would be left thinking "she looked so .., what?"

Solution 2:

Both usages are valid in this instance, although there might be some out there who would find the usage of "so" at the end of this sentence to be awkward, expecting the sentence to continue.

For a more pronounced difference between "it" and "so" and how they are not always interchangeable, compare "I think so," and "I think it."

Solution 3:

"So" is idiomatic replacing a "that" clause thus:

He said that he was happy => He said so.

Similarly "think", "hear", "guess", "suppose", "expect".

It is much less idiomatic for an "as if" clause

She looked as if she was sorry  =>?  She looked so.