You absolutely should list Newspapers.com as the source of where you found the article. A source is where you found the article as well as the article itself. Most commonly we say the name of the newspaper and where accessed, "The Denver Bee, 15 January 1919, 'Obituary of So and so,' accessed on 8 September 2020 at newspapers.com/url"
Another reason for listing a source is so that others can find it. By listing it, other relatives or researchers can find it for themselves, either through their own account at Newspapers or by posting a pullup request on G2G. If you don't list where you found it, the other person may spend hours looking around not able to find it if they don't have a newspapers.com subscription.