well if the caterpillars are feeding on the leaves then yes they will weaken the plant. depends which species it is and if they are protected. If there are only a few then squish them. if there are lots then you may have to resort to spraying with insecticide. check that what you use is suitable for the crop you are growing and spray late evening when fewer bees etc are flying.
hope this helps.
you may also get advice from people in you own area and they are more likely to be right for your area of the USA.
welcome to GoY too :o)

If you are still growing tomatoes at this time of year then you must be in the southern states.
You can spray with BtK if you want a more environmentally sound solution. This is a bacterium that infects the caterpillars and stops them eating. It is harmless to humans. It should be available at your local garden centre or Lowes etc.
Are they big caterpillars with a "horn" on their tail end? If so these are tomato hornworn caterpillars and will defoliate your plant in very little time. You will need to get rid of them quickly but those are easy to just pick off as they are so big.