Is there only one bus daily from Lugano to Tirano?
Yes, I just checked on www.sbb.ch. There is only one bus per day. This is a very scenic route along lake Como.
The alternative, is by train changing at Monza.
What time of year were you going to be travelling? According to the schedule, the direct Bus from Lugano to Tirano only operates from 28 March to 25 October.
That trip is indeed very scenic in parts, especially as it travels along Lago di Como. They make one "rest stop" for about 15 minutes so passengers can use the WC and grab a coffee (I can't recall which town they stop at). If you're going to the rail station in Tirano, as I recall, there's a short walk from the bus station.