Firstly, I would say that all the accommodations on campus at Exeter are great, and I know people who have had positive experiences at all of them. The most popular accommodations are Lafrowda, East Park and Birks Grange - they are the biggest so there are lots of opportunities to meet people (though this is true at all the accommodations). Birks Grange has both catered and self-catered options, whilst Lafrowda and East Park are just self-catered, so that's something to consider. Also, you can select en-suite or shared bathroom as a preference, which will make a difference price-wise too (in my experience, sharing a bathroom is really not as bad as people think as you'd have a cleaner and you're not usually sharing with more than a couple of people).
You can find out more information about each accommodation, see a virtual tour and have a look at prices here: https://www.exeter.ac.uk/study/accommodation/ . There's also an accommodation blog available through this link that will give you more insight into living on campus at Exeter.
I wouldn't say there are any to 'avoid at all costs' - obviously, some accommodations are more modern whilst some are a bit older, but they all have the same facilities and ultimately it just comes down to whether you want to be catered, self-catered, have an ensuite or shared bathroom, and the amount of people you'd want to live with.
To answer the other part of your question, at Exeter we have a diverse student body and people come from lots of different backgrounds, so I haven't found this to be a problem. There isn't an accommodation that is just full of one demographic, every flat and block is varied and that is what makes student life so exciting here!
Beyond accommodation, we have over 300 sports clubs and societies to choose from so you will always be able to meet loads of people with similar interests to you which is amazing!
I hope this helps and please let me know if you have any other questions.