Ya gotta torture your fav/cool characters, it just makes fanart more amusing. I do the same thing with my favorite characters in my fanart (I love drawing the characters complaining or arguing with me, I guess I love breaking the fourth wall XD)
It is a joke referring to her age. When some things are old and decrepit, they tend to get dusty if not looked after thus the pun of saw dust for when you saw wood yow make tiny pieces of bard called saw dust. In other words, she is hitting on vegeta and he imagines that she is so old that she is full of dust instead of life.