Maybe it would help to think of おもしろい as a word outside of English context of interesting/funny. おもしろい was not made to be connected to the English meanings. Just like ことば can mean word, phrase, or language - it means all of those things but in English they are pretty different! So if you say to someone that something is おもしろい, he/she won't be questioning whether it's interesting or funny, they will understand that it's おもしろい! This was difficult to explain, but I hope you can understand!

I guess it depends on the context. If they say it with a smile on their face or more serious countenance. Often people will laugh and say おもしろい after it. Or after they laugh they'll say。。。 うける (in Tokyo etc). or わらける (関西弁） Eg. ハハハ、超うける。　 or ハハハ、めっちゃわらける。