and I quote:
SQLcl: Release 220.127.116.11.177.0246 RC on Thu Nov 12 19:10:40 2015
Copyright (c) 1982, 2015, Oracle. All rights reserved.
Oracle Database 11g Express Edition Release 18.104.22.168.0 - 64bit Production
SQL> COMMENT ON MATERIALIZED VIEW "JANM_CRMACC" IS 'snapshot table for snapshot CERBERUS_DEV.JANM_CRMACC';
Comment on materialized view IS succeeded.
With or without the quotes around the MV name.
This was created by impdp with SQLFILE=file.sql option.
From SQL Dev or command line (naturally).
SQLPlus reports "Comment created"