You have to feed records into Mongo one per line. Like this:
find artists -name *.json -exec perl -p -e 's/\n/ /' '{}' \; -exec echo \; | mongoimport --db tate --collection artists
find artworks -name *.json -exec perl -p -e 's/\n/ /' '{}' \; -exec echo \; | mongoimport --db tate --collection artworks