Was the Monroe Doctrine a justifiable policy for the United States?
In theory, the Monroe Doctrine was much more justifiable than in practice. In addition, it was a much more justifiable policy in the context of the 19th century than it seems today.
On its face, the Monroe Doctrine is eminently justifiable. All that it says is that European countries should not come in and try to colonize the newly independent countries of Latin America. It does not say that the United States should be able to dominate those countries or to interfere in their affairs. This is completely justifiable. However, the doctrine has been used to justify US interference in the affairs of other countries. It has even been used to justify occupations of those countries. This is much less justifiable.
We must also note, however, that such domination of other countries was far from rare in the 1800s. While such actions would be met with great disapproval today, they were commonplace in their time. Therefore, we should be at least somewhat careful about saying the policy was unjustifiable since that involves imposing our values and attitudes from today on people from the past.
It's important to view the Monroe Doctrine in the context of its time. First stated in 1823, it declared that the United States would not tolerate European nations making further colonial inroads in the Western Hemisphere (and setting up in the Americas a U.S. zone of influence). From the perspective of Nineteenth Century statecraft, this sort of foreign policy was not entirely unreasonable on its own merits—European Great Powers were unapologetic when it came to monopolizing their own territorial interests and zones of influence after all. What complicated things, however, and what (from a European perspective) would have made it more unreasonable, was the simple reality that, during early nineteenth century, the United States had very little real political capital to spend in support of that claim. Ultimately, you must keep in mind that the U.S. at this point in time was still a fledgling nation, and it therefore lacked the political clout necessary for European nations to take this kind of policy claim seriously.