What kind of use is your image made for?
Borders and Boxes?
Than you may want to set the Draw Type of the Image Brush to Border or Box and set the margins to get a sliced image.
Plain image?
Can only stay sharp when drawn at the exact same size as the image is.
Every up- or downscaling results in sharpness loss.
If you desire to make a more pixep perfect look, set the image group settings (double click on the texture the content browser) to “unfiltered 2D”.