Sikasso is significantly wetter than the rest of Mali, with a real rainy season from June to September that can leave the rural roads muddy and the unpaved tracks impassable. The best months to visit are November to February, when the harmattan haze settles in but the days are dry and cool by Sahelian standards (around 30°C at midday, much cooler at night).
March to May is mango season — worth timing for if you don't mind the heat, which climbs above 40°C. Avoid August in particular, when rain is heaviest and many side roads turn to clay. Note that the security situation in southern Mali has deteriorated in recent years; check current advice before travelling.