When electricity demand is highest the transmission system is under strain.
And there may well be dirtier or higher-carbon fuels
generating the electricity to run your dishwasher. In the UK peak is typically early evening 4pm to 7pm. Don't run anything that you can do without then, including the dishwasher! Conversely, if you can run the dishwasher in the wee hours, maybe 1am to 4am, less CO2 is likely to belched out to run your wash, in future maybe even zero when wind (etc) is meeting demand.
(Cold rinses take so little energy that you should probably do them when you need them, see below.)