Some solid ceramics and polymers can have very low conductivity, probably because they are made up of very tiny grains compacted together. Heat would not be easily transferred through boundaries between grains. It depends which types of solid you are talking about. But if you are comparing the thermal conductivity of fluid with metal, then it is definitely lower. Depends on what fluids and what solids.
E.g. mercury has good thermal conductivity, much better than cork or wood.