Low ranked master's before PhD
I'll guess that the university in question is still an R1 university (and in the US) and is about mid way through the rankings of those. From that I assume that the standards are high and that you should get a fine education.
And, from comments, you probably need a solid grounding in the new field, so I don't really see any down-side.
A small program can be a bit of a problem, however, if it gives you fewer options. Alternatively, it might also give you the opportunity to work more closely with a faculty member or two rather than being part of the &quot;mob&quot;.
However, don't make assumptions about what is required for admission to a US doctoral program. People frequently choose a different field than their undergraduate major. The typical program is heavy on coursework at the start and often not a lot different from what you do in a masters. So, I'd suggest that you get a solid understanding of the real requirements.