Can a having a fractured skull as an infant cause lower iq later in life?

Depends: The crack itself does not mean a kid will suffer diminished performance later on. Some babies have relatively simple fractures from falls or incidental trauma & don't mess up the brain inside. Some may suffer brain injury to a minor or major degree & the potential outcome depends on the amount & location of brain injury involved. The quicker the recovery the less likely an impact on iq.

Not necessarily: It would depend on the extent of injuries. Was brain matter involved? Was there bleeding in the brain that may have damaged the brain? Also depends at what age this trauma occurred. The brain is an amazing organ and trauma in infancy has a much better outlook than a teenager with similar head trauma.