Is there a word in Chinese to mean "mostly"? Is "大多" appropriate?
is there a word in Mandarin Chinese to mean "mostly" ?


Of course, but that just depends on different kinds of senarios:

1) “相当，非常……”（程度副词）: adverb something related to the depth:

I mostly love peace (我**非常**热爱和平)

2）It's mostly bad=It's quite bad=It's awfully bad 糟糕**透**了！（implementation to describe "bad"'s depth in Chinese, but that's used as an adverb in English).

3）包含很大程度上的可能性，肯定：
Mostly speaking, It's almost of no hope to save people who sunk in the sea disaster of Korea.（可以这么说，救援韩国海难沉入水的群众希望破灭了）。