Database
[{"mfe_average":[[true,5.352702824879613],[false,3.2361364317753383],[null,2.675027181819201]]}]
Query
db.collection.aggregate([{$addFields:{mfe_average:{$map:{input:"$mfe_average",in:[{$cond:[{$arrayElemAt:["$$this",0]},"Valid","Invalid"]},{$arrayElemAt:["$$this",1]}]}}}}])