The first 555 gives an output where the PERIOD is proportional to the capacitance of the level probe.
The second 555 gives an output where the voltage is proportional to the FREQUENCY of the output from the first 555.
PERIOD is the reciprocal of FREQUENCY.
A graph of y = 1/x is inherently non-linear, which is exactly what you are seeing.
How to fix it?
Either use the PIC timer functions to measure the period of the output from the first 555.
Or, if you really want to use the analogue input of the PIC, make the first 555 a fixed frequency, and use the capacitive probe to vary the timing period of the second 555.
JimB