Is electrical impedance at its maximum or minimum, at the acoustic resonant frequency of the material?
It's both. Just like regular crystals, piezo exhibits resonance and anti-resonance within a small part of the spectrum. See this for example. At one resonant point it will act like a series resonant circuit and appear like a short circuit and at the other point it will be a parallel (high impedance) circuit: -



See also this tutorial from Maxim - it contains the basic electrical model of a pizeo and note its similarity with a regular crystal: -