''Majoritarily'' é uma palavra bem menos comum e mais formal do que "mostly". No dia a dia, "mostly" é muito mais natural e utilizada.
Exemplos de uso (Merriam-webster):
He gets around mostly [=mainly, chiefly] by car. [=he usually uses a car to get around]
“What did you do this weekend?” “I worked in the yard, mostly.” [=I spent most of my time working in the yard]
I concentrated mostly [=mainly] on getting good grades when I was in college.
You know what, I think it is good news for Canada that for the first time in a generation Quebecers voted majoritarily for a federalist party. [torontosun.com]
Bons estudos. Compartilhe.