The orientation ans size/shape of the core/cells in the clay tile is a factor. Clay tile can have local variations in the configuration and method of construction. Mortar strength is usually not a factor.
There should not be problem with the wall compressive strength for a crawl space with minimal difference between interior and exterior soil levels.
The 6" thick wall could be a code problem, but 6" walls can have very high vertical capacities. - I have seen 20 stories of 6" unreinforced/partially reinforced concrete block.
Dick