I read one can spray diluted fish fertilizer on the leaves of veggies or fruit trees for nutrition.

Is this method good or should I fertilize though the roots?

Also is it too late to fertilize for this season? If not when is the cutoff date?

There are those that claim that plants can be fed by foliar application and there are those that claim the research that appeared to demonstrate that this is flawed. Since most of your foliar spray will end up on the ground anyway start there.
If your soil is improved and in a good, healthy condition then there would be no need for the fertilizer.