The answer to this question is yes, most of the time,
US dollars are accepted by Canadian retailers, service businesses but not all will accept them.
Some retailers that accept US currency only do so at “par” or at an exchange rate that is lower than what you can get from a banking institution.
Your best bet is to use a valid US Credit card such as Visa or MasterCard where and when you can, and cash for when you can’t.
To get the best rate of exchange, bring your US currency and get Canadian money from a Canadian chartered bank first, then spend the Canadian funds at retailers and service businesses.