Here are my recommendations: There are way more important cards than legendaries, so be sure you have a bunch of basics. Once that's checked off. If all you play is aggro, Mukkla is fine. Tech legendaries are not so good for your first buy (Thalnos, Slyvanas, tBK, Harrison, Hogger, Malygos, Baron, ). You really want to find a card that fits your style, so Mukkla could be a great card you're planning on playing Crusher's Deathrattledin, or another early game deck.
There are a few super valuable and class-specific legendaries. Cairne is a great value card, but his trade off is he is super slow, where as Rag has instant value, he's great to play on a 50-50 board, he's a huge threat and requires resources to kill off... whereas Alex can set up and end game fantastically, but he takes a lot of patience, and typically needs the help of other tech legendaries. The same can be said about Ysera, especially since they use a full turn very late in the game. Then we get into class-specific legendaries which are super powerful. Al'Akir was one of my first beloved legendaries, because he can dish out up to 26 damage with a perfect setup (double flametongue, double rockbiter) but with Savjz's shaman it isn't required, and Savjz's deck is awesome. Tirion is also, super valuable, hes one of the cards I'm lusting for right now. Either he's silenced or he just a massive beast. Jaraxxus, Grommash, and Cenarius are all great, but the decks to utilize them fully are very expensive, I'd shy away from them for now.
As for your question, Mukkla actually isnt a bad way to spend your dust. I'm actually really enjoying the deathrattledin myself, so you're not crazy. Good luck mate.
I think that King Mukla can actually be a decent, sneaky, early craft, but understand that you are committing to aggro for a while by doing this. This isn't a bad idea, because aggro is a good way to farm gold and build your collection, but it can be boring to pigeon hole yourself so early.
I ended up crafting Cairne Bloodhoof. It ended up being the ones who said "King Mukla is fine, but you are committing to Aggro decks if you craft him" that swayed me away. I do enjoy making lots of different decks, and I feel like I can throw Cairne into just about anything. I could not picture myself ONLY playing aggro in the near future.