It depends hugely on the qdisc on use.
For most qdiscs we need to parse from text based input, for cake there is a table. (for layer_cake a multi column table of tins in use)
I think that the dropped/total number of packets is maybe the most useful metric telling us how much of the traffic is effectively shaped. That collected every 30 seconds?
qdisc specialists like @tohojo @moeller0 @dtaht and @ldir might have valuable input on getting tc stats of SQM actions.