Q did say that Picard and company had no idea what lay ahead. At the very end of the episode, Picard and Guinan are playing that funky chess game, which I think has name but I don't know it, and Guinan says that since the Borg now know about the humans, and Picard finishes by saying that they'll be coming and adds, "Perhaps we needed a kick in our complacency." Q was teaching them a lesson because he did not like hearing such bravado coming out of all the crew's mouths when he said that they weren't ready for what lay ahead.
Now, could Starfleet have been better prepared if Q hadn't interfered? Well, I'd say that's something to ponder. The Borg nearly destroyed the Federation in "The Best of Both Worlds, Part II". The incredible loss of life at Wolf 359 was bad enough, but the Borg were that close to wiping out humanity, assimilating them into their collective. Would it have been better for Q not to have interfered and the Federation to keep on going until the actually ran into the Borg? Remember this too, in the Star Trek: Deep Space Nine
episode "The Jem'Hadar", the Jem'Hadar soldier said that the Dominion was not expecting to meet the Federation for another 200 years. The Gamma Quadrant is technically closer than the Delta Quadrant in terms of areas the Federation has explored so the Federation may have been ready for the Borg since they most likely would have met the Dominion first.
BTW, the next episode to star Q after "Q Who?", where Q "tosses" the Enterprise
into the Delta Quadrant, is "Deja Q" where Q has been kicked out of the Continuum. Another Q advised that he be thrown out because of all the damage that he has caused to other lifeforms. One of those charges against Q, of course, is the interference with the Federation by placing them directly in the Borg's path.