how am i supposed to communicate that something wasn’t funny?

i'm autistic and my entire life have struggled with humor. i have been told i'm a naturally funny person, but i also seem to have high standards for humor when it comes to other people, especially with jokes that are harmful or at other people's expense. in social situations irl, i'm rarely able to communicate that something someone says wasn't very funny or was hurtful, and so instead end up looking at them with a straight face which ruins the mood a lot of the time. i also do not like pretending that the joke was funny, as it might give off the impression that i enjoyed it and will cause it to happen more frequently. so how can i express to someone that something wasn't funny, preferably subtly?