Hi,
I totally get the worry, when I was applying for universities I was also asking about the diversity of Cardiff as well! It was definitely something I worried about when applying and a factor that I took into consideration when applying for universities.
I'd say honestly, diversity is decent here! There's a good mix and representation of students. The Student Union is pretty good with it as well, there's loads of
cultural societies here, and you can tell the uni really values the diversity here - there's often events running like the annual Go Global (a show night where societies show off performances from their culture), or the LGBT Showcase.
There's a good community of international students, and I've never really thought about it lacking in diversity, if that makes sense. The diversity outside of uni is also decent, and it shows for example on places on campus like City Road, which is basically just a long road of international restaurants and food places

Let me know if I can help answer anything else!
~ Fatiha, Cardiff University Student Rep