What should I do if the plastic waterline connection on my dishwasher is stripped?
The blue plastic piece itself can't be replaced. The whole inlet valve needs to be replaced. Pretty sure it's this part which costs about $50. Not very familiar with taking apart dishwashers down that far but the appliances I have taken apart have been surprisingly easy. I've always found videos online that helped. This video looks like your dishwasher. https://www.youtube.com/watch?v=MNMd_lDdG_w looks very easy... but then again so does screwing in a supply hose. :)

Considering it's a new appliance and your inexperience with repairing dishwashers you may want to call Whirlpool. It may even be covered under warranty.

Very surprised to see that it's made of plastic.