When someone says you're are smart as a fox it's a good thing and means the person thinks you're very smart, intelligent etc.
Foxes are pretty smart and intelligent so when someone is smart as a fox they are saying that you are very smart.
Being smart as a fox means that one is very intelligent, similar to the romanticized personality of a fox that being, that they are sneaky and cunning, suggesting that they have high intelligence.
I used to take it the wrong way when someone told me as a kid that I was smart as a fox but later found out that being smart as a fox was a good thing.