Hello Good Evening Pelin,
English is very nuanced and the two sentences could have different meanings:
I like the pattern on your t-shirt refers to the picture/image such as a logo or design.
I like the pattern of your t-shirt refers to the tailoring of the t shirt in terms of the pattern design of the width, collar, seam and cut.
That's my view.
Precise use of words is essential to convey meaning.