I think the way Mrs Hall is portrayed is okay. The series is set in the early 1930s and even then it would not be that uncommon for a "professional" like a vet or Doctor to have a servant to help with house/kitchen (usually in the form of a "daily help").
Remember Mrs Hall takes an "expanded" live in role as Siegfried as is widower and the helping with the surgery/telephone would have often have been done by the Vet's wife and as he would be attending out of hours calls etc a live in "help" who did cooking etc was probably essential. You could imagine that with just the the two of them (occasional short lived assistant) they could be more like friends rather than master and servant.
Obviously Siegfried is from a well off, middle class background (able to fund his brother, pay for his cars and large surgery etc) was a RCVS officer in WW1.
All creatures great and small (TV) is actually set at quite an interesting time in the 1930s, there was a great deal of rural poverty (world depression) and farms struggled - it would have been difficult for some farmers to pay their vet bills and it would have been important for vet's to develop their more "lucrative" work such as pet and especially non agricultural equine work.
Lots of farms in the 1930s would not have been that different to farms in say the early 1900s with only hints of modernisation due to the depression.
What "saved" and changed agriculture was ww2 - farmers had MUCH more money by the 1950s (especially those who owned their land vs tenant farmers) and the pace of change to mechanised farming was massive. This period is known as the second agricultural revolution.
Alf Wright started work in 1939 so would have seen these changes first hand and even at this time, some vets worried about the change farming to a more industrial industry and the impact on animal welfare.
Post ww2 also saw changes in land ownership with big estates broken up and some tenant farmers getting ownership of the land they farmed, but lots more under less benign landlords too.
Apologies for the essay!