I think the translation مُمتِعٌ = fun is kind of misleading. "Fun" (in English) is a noun and used colloquially as an adjective ("He is fun."). The Arabic word مُمتِعٌ is an adjective and not a noun. It can mean a lot of things - however, "funny" is not at all the first meaning that comes to my mind. I perceive ٌمُمتِع as a word to describe something "interesting", "pleasant" or maybe "entertaining" in certain contexts.
That's just fine. "Fun" usualy means a mix of "entertaining" and "pleasant", with a touch of "interesting" as well since it's definitely not "boring". However it does not necessarily mean "funny", which generally implies humor. In short, "fun" makes you smile, while "funny" make you laugh.
First of all, I am sorry I couldn't get my message across ... but I fear it is not just fine. The problem is that مُمْتِعٌ is NOT a noun. "Fun" however is (it doesn't matter that it is colloquially used as an adjective). You can't use مُمْتِعٌ like the English noun "fun" in other contexts. It's like trying to teach people the word ٌمُشْمِس ("sunny") by telling them that it means "sun". It's a mistake and you won't be able to use the word correctly in Arabic. For the rest, you confirmed exactly what I said: "funny-ha-ha" is not an option - so I was trying to suggest alternative translations (that are real adjectives).