Bumbry struck out in 12.6% of PAs, which was almost exactly league average in 1980, probably a bit less than average across his career. He had an ISO of 0.096, compared to a league mark of 0.122.
Mullins is striking out 20.2% of the time, in 2021 the league mark so far is 24.1%. ISO (career) for Mullins is .140 vs. league of .171.
In context Mullins strikes out about the same and has more power. Mullins may not hit for as high an average, but the way baseball is trending few will. We have a long way to go in '21, but right now batting averages are the lowest in all of MLB history. Lower than 1968, lower than the deadest years of the deadball era.
I don't think either is tradable this season due to injury concerns and short track records, but if they both look good in a year, Hays and Mullins could have a fair amount of trade value either in the offseason or the 2022 trade deadline.
Having two plus defensive centerfielders is a luxury, but if McKenna hits enough to be a a good fourth outfielder, it is worth considering trading either Mullins and Hays to upgrade another position (third, short, or second).