If you buy the city pass, do you still need to reserve a time or is it 1st come, 1st serve?

We did not reserve our tickets beforehand, but used Sighteseeing Flex Pass to redeem tickets on site. There was a queue but not very long one (a couple days ago).

You reserve a time when u book ur ticket

Yes

You do not reserve a time you just go there

You don't need a reservation. When we were going there, there was not a long line either. The entrance is on the 4th floor of the Mall