Meaning of 纷纷 (fēnfēn) referred to 雨 (yǔ) In the famous poem "清明" by the Tang dynasty poet 杜牧 (Du Mu), rain 雨 is "纷纷".

As reported by dictionaries, like https://www.yellowbridge.com/chinese/dictionary.php , 纷 (fēn) means "numerous; confused; disorderly", then 纷纷 (fēnfēn) is one after another; in succession; one by one; continuously; diverse; in profusion; numerous and confused; pell-mell.

Referred to rain, 纷纷 means that rain is "continuous and long-protracting, with not much water falling all at once, maybe not very windy, somehow boring and sad" (this would be my interpretation, according to the poem) or "copious, with lot of water and wind, stormy, somehow violent"?

Is "雨纷纷" an expression that could be used in everyday life, or only an aulic expression?
Baidu Baike's page on 清明 has a vernacular translation of the line:


  清明时节雨纷纷


which goes:


  江南清明时节细雨纷纷飘洒


Here 雨纷纷 has been rendered as 细雨纷纷.



细雨纷纷 is defined in ABC as:


  a steady fall of drizzling rain


while KEY defines it as:


  steady drizzle




雨纷纷 itself is very poetic, but it is well known enough that most people would be aware of its existence and usage. The entire first line of the poem, in fact, shares the same status. Sometimes you will see or hear it quoted.

Unless the situation calls for it, there is no real reason you would want to go around sounding like a bard and it would be quite off putting to most people listening to you. 

For normal everyday conversations, 细雨纷纷 would do just fine.