Will the US resident be taxed or need to declare amount of a Canadian Cash gift of approx $20,000 USD?

If your mother is not a US citizen, and the gift is "only" $20K, it seems that you do not need to tell the IRS.

https://www.irs.gov/businesses/gifts-from-foreign-person

If you are a U.S. person (...) who received large gifts or bequests from a foreign person, you may need to complete Part IV of Form 3520, Annual Return to Report Transactions with Foreign Trusts and Receipt of Certain Foreign Gifts, and file the form by the 15th day of the fourth month following the end of your income tax year (generally, April 15th for individuals), subject to any extension of time to file that may apply.

You are required to report the receipt of foreign gifts or bequests only if the applicable thresholds apply. For purposes of determining the reporting thresholds, you must aggregate gifts received from related parties. See the instructions to Part IV of Form 3520 for more information; see also Section VI of Notice 97-34 PDF.

For gifts or bequests from a nonresident alien or foreign estate, you are required to report the receipt of such gifts or bequests only if the aggregate amount received from that nonresident alien or foreign estate exceeds $100,000 during the taxable year. If the gifts or bequests exceed $100,000, you must separately identify each gift in excess of $5,000.