Sumedang is a year-round stop — there is no particularly good or bad season for a town visit of a few hours. The wet season (roughly October to April) brings heavy afternoon rain, which makes the Gunung Kunci walk slippery and the alun-alun muddy, but the museum visit is unaffected.
The town is at around 550 metres elevation, noticeably cooler than the coast but warmer than Bandung's highlands. Weekend mornings are the most pleasant time to visit: the market is active, the tofu stalls are fresh from the morning oil, and the alun-alun has foot traffic without being crowded.
Avoid arriving late afternoon if you want to see the museum — like most Indonesian government-run museums, it closes by 3 or 4pm on weekdays and may have reduced hours on weekends. Check locally before relying on fixed hours.