chitgoks
Just yesterday I was able to buy a Mürren-Schilthorn ticket 5 minutes before a departure in the SBB Mobile app, although the Gondolas were quite full. Other people simply bought tickets at the ticket sales booth at the same time. And if a gondola is full on a rather busy day like yesterday, the next one will depart 15 minutes later, not 30 as published in the timetables.
The QR readers at the turnstiles are also able to read the SBB App QR codes displayed on mobile phones.
Note that the Birg-Schilthorn cable car will be closed after October 13th, as the current cable car (actually all of them) is about to be replaced. All cable car stations between Stechelberg and Schilthorn are currently pretty busy construction sites, partially impacting some views or viewing platforms.