The construction with "wer" would be "Wer langweilt sich?" "sich langweilen" = "to be bored", literally "to bore oneself" Please note: "Wer ist langweilig?" = "Who is boring?"

English "to bore s.b." is "jemanden (Akk) langweilen", so the literal translation of "Who is bored?" is "Wer ist gelangweilt?". However, German has a construction "jemandem (Dat) ist ": Ihm ist kalt. Ihm ist (es) langweilig. Ihm ist es egal. Ihm ist alles gleich. The dative is the case used for someone suffering or benefitting from the action, so you can so why the dative is used. English does not have this construction (because English has lost the dative case, and it has merged with the accusative and nominative). The point "but it would be easier to use the passive" doesn't make sense; for a German, the passive sentence is more difficult and sounds more stilted than the sentence using the dative. And that's not restricted to German, for example Latin has the same construction. English is not German, they have different constructions. If you try to understand German by converting it literally into English, you won't be able to learn German. Just like I couldn't learn Chinese by translating it literally into German.

Literally: To whom is it boring?