The problem here is not the LQ, no ... the problem is the text placement and that you used sharpen on that text instead of using it on the actual render to make it look at least mid quality. Not to mention that some of the text (like the little one on the left side) is not needed. I feel like that you have blurred the down left side of the background too, just a little bit though.
In my opinion you did a good job with the whole CC and with the picking up a great tone (obviously green) for the whole signature. You just need to work on your depth and also of the placement of the text.
Nice job overall and keep up the good work, mate! Likewise, for a first vertical signature you did an outstanding job.