Do temporary plans fulfill the ACA’s individual mandate for health insurance coverage?

No, short term health insurance does not qualify as minimum essential coverage under the ACA. However, since the individual mandate tax penalty was repealed effective January 1, 2019, there is now no federally enforced financial penalty for not participating in the ACA and obtaining a qualifying major medical policy (Note that you could still be subject to a tax penalty for 2018 or earlier if you went longer than 3 months without qualifying major medical coverage during that time.)