Some types of metallic ions in your water can create soap scum which reduces lather. If you make liquid (KOH) soap and want it to be clear, any amount of soap scum will cause the soap to be cloudy.
More importantly, other types of metallic ions can trigger DOS (rancidity) in your soap.
What happens will depend on the type of ions present. Calcium and magnesium ions => scum. Iron, copper, etc. ions => DOS.
Some people use their tap water without apparent problems, but if you do use it and have recurring trouble with DOS, you might consider switching to distilled or demineralized water.