As you already know, I had problems with the vet clinic too and couldn't get it beyond bouncing between 3.5 and 4 stars. I did some googling at the time and a lot of info coming up was telling me it could be an ambiance issue - which it was sometimes because pets were making mess faster than I could clean it up, but as far as I could tell decoration was fine (I bought the premade clinic in Brindleton Bay). I had 6 robot vacuums that were set to run every 2 hours during the day, but they were unreliable and would miss the mess 50% of the time. I had my vets clean it between patients but there's only so much they can do.
As it happens though in the end of day summary sheet it often didn't label ambiance as an issue, except on the days in which it seemed like every single pet was being sick everywhere. Mostly it named quality of treatment and value of service as issues. I changed the options to the top quality treatment and knocked the price down to 75%, the lowest possible, and the quality of treatment was still an issue (the little info box in-game told me to increase quality and/or lower prices, and I did both to the max and it didn't help). As for value of service, I just diagnosed the pets and treated them, I'm really not sure what else I could have done. As soon as I knew the illness I treated it correctly and didn't waste more time diagnosing, I never administered the wrong treatment or anything. Perhaps I should have been treating them by eye without bothering with diagnosing? I haven't played the vet career long enough to know the illnesses by the symptoms I can see as well as their required treatments yet, so if I had done that I would have been giving wrong treatments all the time. I also did the "call a good pet" action after every single treatment.
Oh, and in the last couple weeks I had 3 pets die at the clinic which I'm sure didn't help ratings haha.
All that being said it was fun before it became frustrating and I hadn't had a chance to play with vets or restaurants before this, so thanks for this challenge.