When defining your materialized view, the columns in your SELECT are not matching the columns in the destination table. You are simply missing an AS mtm_sum clause in your view:

CREATE MATERIALIZED VIEW crypto_trade_mtm_aggs_view
TO crypto_trade_mtm_aggs_dest
AS SELECT 
    counterparty, 
    sumForEachState(mtm) AS mtm_sum
FROM crypto_trade_mtm
GROUP BY counterparty;