The best time to visit Mahabaleshwar is October to February — cool, clear days with views from the plateau rim stretching for 50 km or more on good days. The strawberry season begins in November and runs to April. March and April see the heaviest weekend traffic as Pune and Mumbai residents escape the lowland heat, and accommodation prices spike over long weekends.
May and June are increasingly crowded and hot (temperatures reach 25°C, warm by hill-station standards), with the summit season ending abruptly in June when the monsoon arrives. June to September is the monsoon: extreme rainfall (Mahabaleshwar receives around 6,000 mm annually, one of the highest in India), thick clouds, closed viewpoints, and leeches on the forest paths. Many resorts close entirely. The landscape is gloriously green and the waterfalls are in full flow, but visibility from the famous viewpoints is essentially nil. Some travellers love it; most don't.
Avoid major Indian public holidays (Diwali, Holi, Republic Day, Independence Day) if you prefer quiet — the hill station fills completely with domestic tourists and prices double.