There isn't any thumb rule on maximum ripple voltage. When CPU is operated at 24MHz, the PSoC's VDD has to be greater than 4.75V. The ripple content in the supply should be low enough such that the voltage does not droop below 4.75V. Also, for better analog performance it would be better to have the least ripple.
Refer the device specific datasheets for any specific constraints on the supply voltage.