What do profs actually think when writing a letter of recommendation for an average student for grad school

Grad school application starts now along with all the anxieties associated with it.

One of the question I ask myself and repeatedly get asked by my peers is that what would the profs think when the transcript and resume reveals that they are just an average student (i.e. 3.3 GPA, at minimum of grad school acceptance requirement).

Of course, the top factor here is that we are being evaluated by people who are exceptionally talented and likely the top students back in their days. And plus we may have built an amicable relationship over a long period, no one wants to appear like a useless person. Lastly, there may exist a significant time period between when an application is sent out to when the letter is received, during this time I find a lot of students will question whether the profs are still open writing the letter for them.

What if a professor doesn't like what he sees in the transcript, or feel that the resume does not reflect any exceptional talent, what would a prof usually do in this case?