Kelp is a very low efficiency smelting material : r/Minecraft Reddit
There seems to be no reason to use kelp since even charcoal outclasses it: It takes 90 seconds of fuel to smelt the 9 dried kelp needed to make a dried kelp block worth 200 seconds of fuel for a gain of 110 seconds, or seconds of fuel per kelp. It takes 10 seconds of fuel to smelt the 1 wood block needed to make charcoal worth 80 seconds ...