Yes, living in Sao Paulo, I went to either SSA or REC every year, and both airports were surely messy during Carnaval holidays. But in GRU, only the domestic arrivals may be crowded, but nothing too bad, as people don't come back home all at once: some travel on Fat Tuesday, some on Ash Wednesday, and many come back the following weekend.
The departures side will be rather empty, since GRU is not a Carnaval destination.
Same way for the city traffic: chaos on the Friday before Carnaval, then nothing going on during the holiday, and finally some incoming traffic on Tuesday and Wednesday.