Well, I dont know that It would necessarily be frowned upon, because there are some, environmental, social, and cultural issues that would not pose a problem if a ships captain choose to have all of one species of crew,
for example, we know of many species that either require less gravity in order to be more mobile or need special environmental adjustments., we know that Vulcans prefer a hotter environment then humans, and we know that Andorians do not do will over extended periods of time in warm environments, So I would think that in order to accommodate the comforts of the crews, thereby ensuring that they are most efficient that Starfleet would support a all of one species crew, and that the idea of Racism is simply not in their frame of thought.
On a separate note however, I would be interested to see if STO has all of the Xendi races as members of the Federation.