Hi Melvin,
are you sure you are using T=0 when talking to the card? simtrace2 (sniffing as well as cardem) only implements the T=0 protocol so far, as in the SIM card world, there is no need for other protocols like T=1.
If your card/reader are using T=1, then this would certainly explain why you don't get any output.
Can simtrace2 be used to trace any arbitrary T0 card communication or is it somehow limited to SIM card APDUs ?
It can trace any communication using T=0, there is no constraint on the type of APDU/TPDU used.