Are upsc interview dates according to the marks secured in mains?

No. 

Interview dates are alloted based on exam centre codes (The first two digits of roll number). The exact criteria of choosing is unknown. Probably, it is randomised. But, Mains marks is certainly not the criteria.

This randomised allotment ensures that there's a good mix of people coming from different regions every day. You'd not see only Delhi people being interviewed on a particular day! Only a few with their roll numbers begining with '08' would be called. This is what I've observed till now. You may verify this with the interview schedule of the past few years.