First of all there was actually a Muslim population in the country at the time. Somewhere around 15% of enslaved people brought to North America from Africa would have been Muslims, and some estimates put this percentage much higher. Prior to the banning of the international slave trade in 1807 there would've almost certainly been thousands of Muslims in the US. Although actually practicing their religion would have almost always been forbidden or suppressed we do find examples of enslaved people meeting up for communal prayers or passing around excerpts from the Quran copied from memory. There was very little tolerance though of enslaved people practicing Islam, and slaveowners were often expected or even required to make an effort to convert their slaves to Christianity. In fact, converting enslaved people to Christianity was often cited as a reason for slavery to exist. For example, Omar Ibn Said was an Islamic scholar from what's now Senegal who ended up as a slave in North Carolina. He publicly "converted" in 1820 (he most likely continued to practice Islam in private) and his story was widely spread as an example of how slavery was beneficial because it introduced Christianity to Africans.

That being said, when the founding fathers mention Islam or Muslims broadly, they don't usually seem to be considering enslaved West Africans already living in the country. In general they most often mention Muslims as a rhetorical device to express their commitment to religious freedom (i.e. "I am so committed to religious freedom I would even allow a Muslim or a Hindu to take office"). People at the time would've understood this as more of a statement of principle rather than a comment on what would've been a pretty unlikely scenario. Interestingly, William Lancaster, a politician from North Carolina, opposed the religious freedom clause in the constitution because it was so broad as to potentially allow Muslims (or more realistically Catholics) to one day become president, although this is something he sees happening in "four or five hundred years."

Of the founders, Thomas Jefferson seems to have been the most interested in Islam. He acquired a copy of the Quran in 1765 and took extensive notes on Islamic legal traditions. He is ultimately somewhat critical of Islam though, saying it stifled free debate and combined church and state, but as president he participated in an Iftar with an ambassador from Tunisia.

John Adams seems to take an even more liberal view of Islam. He mentioned Muhammad favorably in his book Thoughts on Government as a "sober inquirer for truth" along with figures like Socrates and Confucius. He also signed the Treaty of Tripoli as president, which states that the country had no

"enmity against the laws, religion, or tranquillity, of Mussulmen"

Impressions of Islam were filtered through ideas about the Ottoman Empire, which was the go-to example of an oppressive despotic state at the time. Orientalism was also fairly popular in this era and it could even be considered a bit trendy for educated men to take an interest in Muslim (especially Turkish) customs.

In general the founders, seem to have been vaguely respectful of Islam but never really considered that it would ever hold a major role in America. For the most part they envisioned America remaining overwhelmingly white and Protestant. They would've probably shared to some extent the general bias that Muslims were "heathens" compared to Christians, although the founders tended to have much more unorthodox religious beliefs than the average person and less subject to this.