Mushroom card icon based on state

Is it possible for the mushroom card to display an icon based on a sensor state?
My current setup doesn’t work:

type: custom:mushroom-climate-card
entity: climate.radiator_woonkamer_voorzijde
show_temperature_control: true
name: Voorzijde
icon: >
  {% if is_state_attr('climate.nefit', 'hvac_action', 'idle') %}
  {% else %} 
  {% endif %}

The radiator is a TRV and I would like to base the icon on the state of the central heating.