Probably both. Increased body mass index (being overweight) is linked with chronic plantar fasciitis (PF). But so is having a flat or low arch. These two factors together are enough to result in plantar fasciitis.
Nerve damage from diabetes called diabetic neuropathy has also been linked with PF. It’s not clear if the increased rate of PF in patients with diabetes is caused by a systemic change from the diabetes or a local change from mechanical factors. Perhaps it’s a combination of both.
The fact that thin people without diabetes can develop PF suggests other factors as well. There may be differences in the thickness of the fascia that we are born with that contribute to the problem. Most likely it’s a multifactorial problem meaning that many things come together either over time or at the same time to result in this condition.