Smartforms can also be debugged, or rather: the Abap coding that is backing the Smartform can be debugged. To do this a bit of background on how Smartforms are composed is relevant. When a form is composed, abap logic can be involved in a few ways: the initialization block of the smartform, a routine pool, but also the coding blocks which are set up in the Smartform itself all play along. All of this Abap logic is held in a generated function module, which is available for debugging purpose like any other function module.
So all you need to do is determine what the generated name for the Smartform module is, which is available on the SMARTFORMS: start the transaction, enter the name of the Smarform and go to the SAP form builder main screen. Click Environment => Function module name. An information message with something like /1BCDWB/SF00000127 will be displayed. Now start SE37 and fill in the module name. Look for
PERFORM %GLOBAL_INIT
and put your break-point there, or even better: look up the bit of coding that you wanted to debug.