A dishwasher needs 120 degree water to dissolve the detergent and to sanitize, 140 degree. As Terry suggest, most new dishwashers do have built in water heaters and the more important, it maintains the temperature during the wash cycle. A point of use WH do work well but the water temperature will not be maintained during the wash cycle. I have a BOSCH DW and it heats the water during the wash and rinse cycles. It has a Sanitize option button, Whirlpool has a High Heat option.
Sometimes the complaint of the dishwasher not cleaning is the spray arm nozzles get clogged with food debris. My daughter-in-law had the complaint and how they need a new dishwasher. After few hours taking apart and cleaned out the nozzles, mosty paper residue from labels that are put on dishes or glasses, it performed like new. Many new DW's, i.e. BOSCH, have a filter screen inside the tub to prevent the nozzles from clogging up.
So your options are replace the dishwasher or use a point of use on the hot water line. You can also tee off of the from the WH to feed the sink faucet. Insulating any pipe is always a good idea if it is in unconditioned space.