Hi again dear MehdiP
I thought about it and searched for and asked ... . Anyway, I make some changes in my answer. (It's better to use "It" instead of "He" because "must-see" usually refers to things (here, the situation) , not people!)
As you said "must-see" has a positive connotation! But consider that "تماشایی" is also positive ! It brings to mind that something (here the person in mud) is some sort of attraction! While the situation (having fallen in mud) is not positive; it's negative! So, I think there's some irony here and the speaker refers ironically to the situation.(I don't know what the context is! Maybe: Hey you should've been there and see him all covered in mud ! ha ha ...) How can we translate the sentence in a way that the ironical part of the meaning be kept? I, personally think in this situation, "must-see" can be a proper translation (Though there can be other nice equivalences and I'll be happy if other friends can suggest better ones.); Unless we change the sentence and say:
"He was a sight not to be missed! All covered in mud."
or just simply say "You should have seen him! ..."
(As most of us know, we can't always translate word-by-word.)
We can also say "He was an eyesore! All covered in mud (from head to toe). You should have seen him!" But "eyesore" is something bad that people don't want to see, so it's not "تماشایی"!
(Maybe we can say "Hey you should've been there and see him all covered in mud ! ha ha ... That was a freak show!" but as it sounds derogatory, we'd better not use it.)
Thank you again.
I'm awaiting for better suggestions.