Your plant looks normal to me. I don't see any curling there. Probably it is watered a bit too much.
As far as the growth vigor, it depend on the light and temperature.
Wait for more comments.

Seysonn

Looks like an ok plant to me. Different varieties have different characteristics and tendencies to curl. Tomato leaves are very cool. I haven't read this elsewhere but I think the tomato plant uses them as tendrils by just observing my own masses of leaves.

Back to yours. There are many reasons a tomato plant can curl that have to do with environment, toxins and simply poor general fertilization rather than a single obvious deficiency. IMO, this picture does not contain enough enough info to really rule in or rule out many of them. I would be sure to not over water the plant and smother the oxygen loving roots, though, so that is my suggestion, though just general. Is that a 240 mL cup? It might be very small and each time you water it , iof there is no drainage as the plant reaches transplant size, is a big change of soil water, especially if it collects on the bottom. This can interfere with nutrient take up as well..

Good luck.

The new leaves on the top look a bit pale to me, and I would think that it needs some fertilizer. Perhaps it also needs more light. You could add some more potting soil to fill the cup almost to the top, or you could transplant it into a bigger container.

If you transplant it, you will be able to examine the root ball and figure out if it is too wet or too dry.

Just my two cents.
Linda

New leaves are typically a bit on the light-colored side.

Thank you for the replies. I was thinking perhaps the tomato plant might be quite stressed out as it's planted in a small cup. I just transplanted it into a bigger pot. Hope it will be much better. :)

Have you fertilized as yet? If not, start.