I don't think 5 is too low as a general rule, but the timing is more important than the order. If a player is your trump card, you should give him a chance to exact damage. Teams shouldn't have players whose job is just to fill the gap between the openers and the lower-middle order (unfortunately, many seem to).
Pollard has predominantly needed 5 to 10 overs to develop a score of 30 or greater. His ability to produce a flurry of boundaries does mean he can be useful to 'pinch hit', but strike hogging becomes a crucial factor. None of his 50s have come from entering the innings later than the 14th over and his two highest scores of 83 and 89 came from starting as early as the Powerplay.
For this game, logic suggests that maybe Pollard could have come in 2nd drop if it was important that he produce a big score. However, it's equally arguable that JP Duminy should be given every chance to make a big score as well.
Nevertheless, the biggest problem for Mumbai was with the ball and in the field.