It's hard to tell if these are early virus symptoms or something else. Was any herbicide used nearby? We don't get the impression that thrips are involved, but you could inspect the leaf undersides and crevices (where the leaves join the stem) to see if any insects are hiding there.

Viruses can cause various unusual yellowing patterns in dahlia leaves, but the symptoms are quite variable and can also sometimes resemble nutritional deficiencies. It's hard to say at this point and the plants may just need a little more time to get established. 

For now, just keep monitoring your plants to see if symptoms worsen or change. Keep the mulch pulled back a little so that it isn't directly against the plant stems, and be sure to water if we get into a dry weather period (feel the soil by pressing a finger down a couple of inches to check whether it is damp or not). If there is less than 1" of rain in a week, you may need to water. 

Christa