Certified GKH Gardening Expert
It is ok to trim your hibiscus plant if you are overwintering it indoors. In fact, this can lessen the stress it will likely experience while acclimating to its new indoor environment, which is why leaves tend to drop off. You can trim whenever you deem necessary, but for hard pruning I would wait until spring when you set it back out, or for hardy types outdoors year round.
These articles should help: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/tips-for-pruning-hibiscus-plants-when-to-prune-hibiscus.htm