I did a quick search on the undergraduate academic backgrounds of MBA students at Harvard and Wharton for the class of 2018:
http://www.hbs.edu/mba/admissions/class-profile/Pages/default.aspx
https://mba.wharton.upenn.edu/class-profile/
Harvard:
Economics/Business, 41%
STEM, 38%
Humanities/Social Sciences, 21%
Wharton:
Business, 26%
STEM, 28%
Humanities, 46%
So, at least for these two exemplary MBA programs, business/economics grads seem to do as well as STEM grads in admissions.