It's nice to have the entire quotation for context:
“The average newspaper, especially of the better sort, has the intelligence of a hillbilly evangelist, the courage of a rat, the fairness of a prohibitionist boob-jumper, the information of a high school janitor, the taste of a designer of celluloid valentines, and the honor of a police-station lawyer.”
I think wfdjr is not far off: she or he said, ''a person who thinks alcoholic beverages should be illegal and who attacks unsuspecting customers of bars in pursuit of that goal".
Mencken often talked of 'boobs,' stupid people (i.e. most). I think it means one of those zealots--e.g. the evangelical types with the pamphlets--that 'jumps', besets, accosts, you on the street. So I disagree that the context must be a bar, and that 'attack' is the correct term: 'buttonhole' would be better.
Mencken included the line in his article "On Journalism", published in the January 1920 edition of the magazine The Smart Set (and it may be noted that the original quote referred to a "Baptist evangelist", rather than a "hillbilly evangelist".) Mencken often used the term "boob", or a variant of it (such as "booboisie) to describe unthinking people who accept conventional ideas. For example, later in the same article Mencken speaks of "the psychology of the Homo boobus, or inferior man -- which is to say, of the normal, the typical, the dominant citizen of a democratic society."