When I longarm quilt T-shirt quilts I never stitch through the decals. I go around them, in & out- however I can to stabilize everything without stitching through the rubbery stuff. I use warm & natural or a like batting that allows up to 10” between quilting lines. Lengthen the stitch length a little bit and smooth it- don’t stretch it tight. Hopefully it will already be pretty well stabilized- you don’t want to stretch the T-shirt fabric and cause distortion and waves .