Does 'Taxable Income' from a Roth conversion count as income source for IRA contribution?

The sum of your (Traditional and Roth) IRA contributions cannot exceed your "taxable compensation" for the year. (Or, for spouses filing as Married Filing jointly, the sum of both their IRA contributions cannot exceed their joint taxable compensation for the year.)

The section of Publication 590-A on the General Limit for Traditional IRA contributions says:

For 2019, the most that can be contributed to your traditional IRA generally is the smaller of the following amounts.

• $6,000 ($7,000 if you are age 50 or older).
• Your taxable compensation (defined earlier) for the year.

And similarly, the one for Roth IRA contributions if you contribute to Roth IRAs only says:

If contributions are made only to Roth IRAs, your contribution limit generally is the lesser of:

• $6,000 ($7,000 if you are age 50 or older), or
• Your taxable compensation.

Note that in both cases, it is not all taxable income that counts, but rather taxable "compensation". The section What is Compensation? describes what does and does not count as "compensation". Although it doesn't mention income from IRA withdrawals or conversions specifically, it does say that compensation is generally what you earn from work, as shown on your W-2, so I don't think it would include income from conversions.