It is CHEMICAL (also called nutritional though plants don't need nutrition, they need proper chemicals and chemistry, they make their own food) deficiency. First, you absolutely have to test the pH. You could have plenty of proper chemistry in your soil but if that pH is off the plants won't be able to use Iron or Nitrogen for instance.
The other thing you should know is at this time your plant is putting its energy into the fruit, not the vegetative growth. If you were to increase the nitrogen right now your tomatoes would get bitter and tough. Your flowers would not set fruit. You'd get great vegetative growth with dark green leaves and NO fruit.
Test the pH. Tell us what you HAVE used how often for fertilizer. What have you added to the soil as well. What type of compost? Coffee grounds and that kind of thing? Ashes? What water are you using? Have you had a proper soil test? Tell us every little thing you've done or added to your soil, please.
I am seeing your tomato plants are in the garden soil and you are using either straw or pine needles for mulch? Your mulches are not decomposed. Anything dead has to be decomposed first and that takes feeding the decomposers who thrive on nitrogen. Decomposition will rob Nitrogen from use by your plants. Do not ADD nitrogen willy nilly or you won't get fruit. Get a pH test, get a soil test first.
Otherwise, when my tomatoes start producing fruit the vegetative leaves get wimpy looking no matter what. This mottling could be very normal OR it could be the beginning of mosaic virus from tobacco. ANY tobacco on hands, the smoke, will transfer this virus to your plants and certainly cause this yellowing. Keep smokers away from your garden plants!
And in that case, Alina is on the right track, a viral disease.