If you want to ship it, go ahead. Nothing is stopping you. Don’t listen to what other people are saying. If you like these two characters together, there is nothing wrong with that.
Now, imo, I don’t see Ash and Koharu together. They don’t interact all that much in order for me to ship them. Besides, I think Goh is a better match for Koharu than Ash is but you do you, pal.