It's only immoral to question the validity of a good cause on the pretense that there's something worse happening out there somewhere...
There are many criteria you could use for making such ranking of problems (a popular one is minimizing dollars per lives saved, or dollars per QALY added). Living in a free society means people are free to decide how much (if anything all) they want to spend on charitable causes, and on which ones. But in one's individual spending, it's worth to think about how to get maximum "bang for your buck", in terms of alleviating suffering.
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[0] - By coincidence, this is on the front page right now: https://news.ycombinator.com/item?id=22993486.