By total numbers but not by percentage of the population.
Let’s say one group has 50 people and 5 commit a particular crime. Another group has 10 people and 3 commit the same crime.
The first group has almost twice as many crimes committed. The first group by percentage of the population is at 10% but the second group is at 30% or three times more likely to commit a particular crime.
That holds true with mass shooters.
By ethnicity the least likely to commit a mass shooting offense is Latino.
By race the most likely to commit a mass shooting offense is Black.
As you said, not that it matters in a particular incident but as compared to population it is a myth that a mass shooter is more likely White. Total number? Yes as Whites make up a majority of the population and more than all other groups combined. As a percentage of the population? No.