Decoupling capacitors on MON pins are recommended. MON pins are ADC channels. There is no internal filtering to average the ADC samples. Therefore, decoupling capacitors are used to remove rail ripple voltage. Also, if resistor dividers are used to scale down the voltages, adding decoupling capacitors can reduce input impedance and improve ADC sample accuracy. 100 nF to 10 nF are good numbers for decoupling capacitor value.
For more FAQ please check the following document.
UCD90xxx Family Frequently Asked Questions and Answers
UCD9090 UCD9090A UCD90120 UCD90120A UCD90124 UCD90124A UCD90160 UCD90160A UCD90240 UCD90320 UCD90320U