Which kinds of t-shirt design get most sell

As you do more research here on T-shirtforums you will find TEXT is the best seller. However, if designs are original and relevant you can climb up the price scale.

In my pet friendly town, dogs and cats 'trash talking' the other sells well.
If it is a "DR Who" convention, 'Trust me I'm a Doctor' will pay well.

IF you are in Key West or somewhere Florida there seems be be room for yet another Jimmy Buffet T-shirt with reference to Margaritaville.

Original art such as offered by Clyde Butcher Studio's carry a ticket of $65.

It is really up to you to find or pick your niche. Being a graphic designer you have an unlimited resource for original design. I suggest visiting FaceBook and Twitter and finding out what your communities are talking about. Then offer a garment where they can wear their opinion AND announce their location with a localized graphic.
By all measures, text such as "Worlds Best Mother" "Soccer Mom," "Pride," "#MeToo," and "I cancelled your vote" are text messages that will pay at the right time.

Hananbd,
What I am saying is T's with a message, (TEXT) sell better than those that are image only.
I would imagine SOCCER MOM will have a ball graphic.
"My vote...." would do well with something patriotic, even if inverted.
"I'm with Stupid" accompanied by an arrow seems to be a favorite.

Now, call to action message should be TEXT ONLY, such as:
"Support your dog's right to vote!" or 'Shhh, text me instead."

As for what happens in the USA, I have no knowledge. However I can read what those who place their products on fulfillment websites say. I do not use fulfillment websites.

What I do know about my local area (this could prove really important to garment sellers) is when I send T down the running trail no less that 110 pairs if eyes see it and when it has a great hook, nearly 40% will visit the website. In the mentioned example I was helping grow a real estate business by offering a special house scoring service. The message was "I scored! You can to. Go to www.....xxxx..xxxx"

TEXT messages on T's are an undervalued marketing resource. Now would be a good time to consider collaboration with local merchants.

Take a moment and think how your great graphic creativity can run with a message like 'It's Just the CONSTITUTION!"

Now you must decide if your art is to provoke and stimulate or offer some peaceful affirmation.