petty vs lowly

petty is a noun but lowly is not a noun.

Word	Noun	Adjective	Verb	Adverb
petty	Yes	Yes	No	No
lowly	No	Yes	No	No

As adjectives, petty and lowly are synonyms defined as:

Other synonyms of petty include junior-grade, lower-ranking, secondary, subaltern.

petty (noun)	lowly (noun)
larceny of property having a value less than some amount (the amount varies by locale)

petty (adjective)	lowly (adjective)
inferior in rank or status	low or inferior in station or quality
contemptibly narrow in outlook	inferior in rank or status
(informal) small and of little importance	of low birth or station (`base' is archaic in this sense)
	used of unskilled work (especially domestic work)