It is true that you can't do a "Clear Counters" the way you can on a Cisco - well, you can but as mentioned it involves reloading the card/LP and it will screw accounting/statistics for that card/node.
What you can do is use a program called EPIC. You should be able to get it through your local Nortel support people. Its a command wrapper that parses commands you normally would type straight into a passport CLI but gives you added functionality like the ability to simultaneously log onto multiple passports and send commands like "d atmif/130+140 vpc/10-20 statistics"
This would tabulate the statistics (transmit cell count etc) on ATMIF 130 AND 140 for any VPCs in the range VPI=10 to VPI=20, on whichever passport/s you're logged into. Kinda nice. You can also do a WATCH on any component/s wich is like a normal DISPLAY but with periodic automatic updating (like every second) so you can look at say a VCC and see attributes like cell counts in rates per second, current values or 'change' values. THIS can be reset which gives the equivalent of a CLEAR COUNTERS without screwing accounting - or needing to reset cards! Ask your support folks until you get EPIC. If they can't or won't, post again and we'll organise some other way to get it to you.