mlterm-im-* が必要だった
mlterm-im-fcitx 入れてmlterm 起動し直しでok に

$ apt-cache search mlterm input method
mlterm-im-fcitx - MultiLingual TERMinal, Flexible Input Method plugin
mlterm-im-ibus - MultiLingual TERMinal, IBus input method plugin
mlterm-im-m17nlib - MultiLingual TERMinal, m17nlib input method plugin
mlterm-im-scim - MultiLingual TERMinal, scim input method plugin
mlterm-im-skk - MultiLingual TERMinal, SKK input method plugin
mlterm-im-uim - MultiLingual TERMinal, uim input method plugin
$ sudo apt install mlterm-im-fcitx