Whether this is a bad decision depends on the goal of offering these cruises.
If the goal is to get people from North America to travel to Australia and take these cruises, then having short cruises with few stops may not meet the goal. But people in North America who want to take a
Disney cruise can take other cruises, which still benefits DCL.
If the goal is to get people in Australia and surrounding areas who have or are willing to fly to North America to take Australian cruises instead, then these cruises may not meet the goal, because the cost is apparently similar to the cost of flying to the US. But people taking cruises in North America still benefits DCL.
If the goal is to get people in Australia and surrounding areas who haven't considered a Disney cruise (because of the distance or because they haven't thought about it) to take a cruise, love the experience, and be willing to take future longer cruises in North America (or in Australia or Asia or Europe), that goal could be met by these cruises. DCL would get customers in addition to those already cruising in North America and Europe, rather than instead.
Given that most of the cruises are too short to appeal to people flying in from far away or to people already familiar with DCL, it looks like the goal is to see if the Australian area has enough new customers to support more cruises in the area. If DCL decides it will, they may plan longer cruises with more stops to also draw in people from other areas. The per-guest-per-night profit is greater on shorter cruises (which is probably why the Wish is doing 3- and 4-night cruises, as the Dream did when it launched).