That's a pretty simple design, not too much that can go wrong with it, I was expecting a long hopper chain where things could get stuck or something.
Does the kelp hopper stay full?
If the kelp is being fed in too slowly so that the smoker gets empty some of the time then some of the kelp blocks burn time will be wasted on an empty smoker.
It would be possible to add a holding chest for the fresh kelp and some redstone that stops feeding in kelp blocks if the chest isn't full. Or add a few layers to the farm so the smoker stays full.
(Or replace the smoker with a furnace to slow it down.)