Platform: Mac Simulator
SDK: 1.12.2
The documentation for the setParameter method on playdate.sound.synth says that parameters are indexed starting at 1, which would be the normal way of doing things in Lua.
Using setParameter, it appears that a two parameter synth like kWavePOVosim will correctly have its parameters mapped to indices 1 and 2. If you pass 0, it appears to behave as though you passed 1. However with setParameterMod, the parameters will be 0 indexed, and modulating at an index of 2 will have no effect.