for the most part vs mostly

for the most part and mostly both are adverbs.

Word	Noun	Adjective	Verb	Adverb
for the most part	No	No	No	Yes
mostly	No	No	No	Yes

As adverbs, for the most part and mostly are synonyms defined as:

Other synonyms of for the most part include largely.

for the most part (adverb)	mostly (adverb)
in large part; mainly or chiefly	in large part; mainly or chiefly
	usually; as a rule