Do I need to copyright all of my T-shirt designs if I have already registered my Trademark with the USPTO?

Both, and neither. Yes, and no. Your trademark protects your t-shirts (and whatever other clothing items you put in your description) as the source of those goods. So you've got exclusive rights for that mark and that class, and that's the way to protect your brand.

As for the short text phrases on the shirts themselves, generally those aren't copyrightable, because no one can own the rights to a phrase. Also, clothing designs generally aren't copyrightable either, ebcause the innovation of the creators is supposed to motivate them to continually invent new things, so those per se aren't protectible either.

Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.