I don't think it matters if the generator is sitting on concrete or not. It's an electrical parameter that is in large part, governed by what kind of generator it is, and what its internal circuit configuration is. If you've somehow got a generator whose THD is dependent on what it's sitting on, I'm afraid you have much bigger issues to worry about.
THD is total harmonic distortion. It is a measurement that tells you how much of the distortion of a voltage or current is due to harmonics in the signal.
The simplest way to view THD is as the ratio of the RMS voltage of the total signal less the RMS voltage at the fundamental frequency, divided by the RMS voltage at the fundamental frequency alone.
The reason you want to minimize THD in the output of a generator, is most important if you're driving a load that has a power transformer as its input. Transformers that are wound on a magnetic core, all have a magnetic resonance frequency that is governed by the dimensions of its core, the type of material from which the core is made, and the construction method used to build the core. Transformers designed for use at lower frequencies (like 60 Hz) are much larger and heavier than transformers designed for use at higher frequencies (like 400Hz). Weight minimization is why many aircraft have 400Hz AC electrical systems, because the transformers are lighter in weight. If you feed the transformer with a signal that contains frequency components (harmonics) that deviate significantly from its design frequency, the harmonic energy is lost as heat in the transformer core. This is in addition to I2R losses caused by the wire in the windings itself. The higher the harmonic content, the higher the energy that heats the transformer core. Done to excess, this excess heat can damage the transformer prematurely.
Inverter generators typically have a higher THD than non-inverter generators, although this isn't always true. This is caused by the switching circuits in the inverter itself. Some inverter generators (like the Honda) have a pretty low THD, because there are extra components in the output network of the inverter to smooth and filter the output waveform, which reduces harmonic content. Cheap inverter generators are often cheap because they omit those extra components to save on parts and manufacturing cost.