It is your choice of course but hardly a common practice. There is no way to know what nutrients that would provide other than some potassium from the seaweed, much less the amounts it would provide. So is it a good idea when compared to the many good fertilizers, both organic and synthetic, that are available for tomato plants? Not in my opinion.

Dave

Tomato plants need the primary elements : N, P and K.

And the secondary elements like Ca(calcium) , Mg (magnesium ), S (sulfur )... and a host of trace elements ( like Na, Fe, Cu, Mn, . ..)

That is why it makes better sense to get a fertilize that is tailored for tomato plants. As Dave mentioned there are both organic and synthetic choices out there.Sure , you can always supplement with various thing.

Sey

I always felt it was wise to let the experts supply me with a balanced/complete fert...rather than me playing mad scientist. I will say that it is kind of fun to mess around with additives...just to see if you can improve things a bit. The guys above have given you good advice.