Is skydiving in IMC (through clouds) legal and safe?
It is not legal to conduct skydiving into clouds, atleast in US. From 14 CFR §105.17   Flight visibility and clearance from cloud requirements:

No person may conduct a parachute operation, and no pilot in command of an aircraft may allow a parachute operation to be conducted from that aircraft—
(a) Into or through a cloud, or
(b) When the flight visibility or the distance from any cloud is less than that prescribed in the following table:



Is it safe? Probably. The chances of mid-air collision are pretty small, but they can happen as is does with aircraft. Midair collisions between skydivers and between aircraft and skydivers are rare, but not unheard of. There has also been reports of near misses between aircraft and skydivers through clouds. So, its better to be safe.