Without the specific diagram of Mozart's Clarinet Concerto in front of me, I can provide a general answer. In classical music, a piece typically begins with a downbeat if it starts on the first beat of a measure. Conversely, if it starts just before the downbeat, it would be considered an upbeat.
Mozart's Clarinet Concerto (K. 622) famously begins with an orchestral introduction that leads directly into the clarinet solo, which typically suggests a downbeat. If you have the specific measure marked on the diagram, look for whether the first sound occurs on the first beat of the measure (downbeat) or before it (upbeat).
If you can describe the diagram or provide more details, I could offer further insight!