If the pumpkin is watery, drain it first in a fine sieve or cheesecloth for 20–30 minutes. That’s better than reaching immediately for starch.
3 If it still needs thickening, I’d use potato starch over tapioca here, and only a tiny amount — around 1/2 tsp, mixed in thoroughly. Tapioca can make cold dips a little gummy or stretchy.