Lambskin is inherently soft, effortless to move in and feels wonderful to touch and wear. It is not a dense, tight grained skin and as such is very soft and flexible. Also, since it is so soft, it is not scuff resistant (does not make good everyday knock-around shoes- cowhide is better here). It does make great fashion jackets that you will be proud of for years.
Lambskin is easily cleaned with a damp cloth, or even more water if needed. Should you have an accident, if the problem iswater soluble- soda spilled on a sleeve for example- it is easily dealt with by getting the problem- soda in this case- into suspension with water and evacuating it from the leather. Don't rub hard and risk physically damaging the skins- use more water. The water will always dry without a mark. On light colored skins it can leave a 'ring' at the edge of the 'what was wet' and dry border. If this happens, re-wet the entire panel- seam to seam- and let it dry. All this being said... Yes, you can wear lambskin in the rain. A rain coat is smarter, but your jacket will survive.