Good Morning Laura, 

Thank you for contacting the U of M Extension Service. 

I am not sure of an aster yellows diagnosis. Here is a link about aster yellows from the U of M Extension Yard and Garden website: https://extension.umn.edu/plant-diseases/aster-yellows

As you read through the information, it indicates one plant could be infected and not the one next to it. It is possible for hydrangeas to be affected by aster yellows, but you might consider the other symptoms of aster yellows:  Do any of the plants with the unusual blooms also have strange or weird foliage growth in the leaves? Of your hydrangeas, how many of the plants actually have deformed flowers? (Following the bloom down to the stem). 

Possibly your hydrangeas are just producing a few unusual blooms due to our wet weather. I would not remove the hydrangeas until you have an accurate diagnosis. 

You could submit a sample to the Plant Disease Clinic for an accurate diagnosis for a small fee. Here is a link to the Plant Disease Clinic: https://pdc.umn.edu/submit-sample

Good Luck!