Another way to view it is that college classes are filled with your future colleagues, and should be treated similar to a workplace. Students should generally dress similarly to how they expect to dress in their future workplaces. Whether that excludes a tank top is a matter of discretion, but clearly the proffesor did not find it to be professional. There’s no hard and fast rule, but I am surprised by the indignant tone in the comments here. Lectures are the domain of the professor and its not outlandish for them to set expectations about what they consider acceptable conduct for their lectures.