The 4.2 inch mortar tube is rifled, yes. Interestingly, the 4.2 inch mortar
round also has corresponding lands and grooves that must be mated up properly with the ones in the tube (a process called indexing). It is very common to see a new mortarman (in his haste to get away from the tube after letting go the round) get the round stuck by not indexing the round properly.
But not all 4.2 inch rounds have the lands and grooves. The illumination rounds for example have none and are dropped straight down the tube while the HE rounds (because of the grooves) spiral down the tube slowly.
The others I mentioned are smooth bore.