As everyone else has said above, the short answer is no, it's not bad etiquette to ask to see the hand - the player showing their hand to someone else is actually breaking the rules.
That said, like pretty much everything else in poker, it'll depend a lot on context. For example, say someone raises preflop and it folds around to the big blind, who looks at their cards, shows them to the person next to them with a "ugh, rags again" look on their face and then folds. Even though it's not bad etiquette to ask to see those cards, some players might still view it as poor form on your part to do so as it's already obvious from the action that the big blind had nothing, and effectively all you're doing is holding up the game.
Sometimes you've got to think about what effect asking to see the hand will have, and what's better for you in the long run. If asking to see the hand will piss a few people off and change the way the game is being played, is that worth the little bit of extra information you get from seeing the hand? If it'll tilt the player in question and have them spewing money, then it's +EV. But if it'll make a fish leave the game, then it's probably -EV.