If your 2017 amended federal tax return claimed itemized deductions rather than the standard deduction, then, yes, at least some, and in most case all, of that state tax refund is considered 2019 taxable income. If that is the case, to determine how much of the refund is taxable you will calculate the difference between your 2017 itemized deductions and your 2017 standard deduction and report the lesser of that number and your 1099-G tax refund. If you claimed the federal standard deduction then the refund is not a taxable item.
The basic idea is that an itemizer is required to list all state and local taxes on their Schedule A and therefore was required to subtract more state and local tax than was actually paid in the end. The IRS handles this by adding back the amount of the over-deduction to income in the year the refund was received.
