Man lands don't make magic fun? I've made a ton of land reliant decks that depend on man lands for the win for casual games only. I don't see how that makes sense. Any card can be considered fun. That's such a subjective term. Who has to consider it fun? In what situations? What card is considered fun all on it's own?
Regardless, this isn't the most fun "casual" card contest. Though fun can be the leading contribution factor, effectiveness should be just as highly ranked. A casual card can still be good. Platinum Angel, Darksteel Colossus, Deranged Hermit made it in previous lists, I don't see why Mutavault can't.
You compare it to Ravager, but Ravager is broken. Mutavault still has places in casual decks for the simple fact that 1. It appeals to tribal 2. It's a manland. Ravager has a very specific set of talents that 1. You can pump it conditionally and 2. It has modular. The modular gives it a pretty specific purpose, Shapeshifter creature type does not. There are decks that can exploit both aspects effectively, but modular is useless without artifact creatures. Not exploiting creature types for Mutavault isn't on the same level of disadvantage if neglected.
As far as the activation cost, what would be a more sutiable amount? 2 mana would be crap, since you could just play factory instead. Even the colored man lands are like 3 mana at most, have abilities, and are larger than 2/2.
If treetop village can be a 3/3 Trample ape for 1G, then I see no problem with Mutavault being a 2/2 shapeshifter. Sure it comes into play tapped, but it also produces green mana itself. Blinkmoth Nexus has pumping, produces colorless, and is a 1/1 flying creature type artifact. Faerie Conclave is a 2/1 flying faerie for 1U, forbidding Watch Tower a 1/5 Soldier, Ghitu Encampment a 2/1 first strike warrior, and Spawning Pool is a Skeleton regenerating 1/1. Stalking Stones and Svogthos both cost 6 sure, but have abnormal effect compared to the others. Nantuko Monestary is a 4/4 first strike for WG and Thresh. And the most similar is Factory that becomes a 2/2 for 1 and can pump. I think that's all the man lands, so it's a good basis for what a pattern can be.
If it produces colored mana it comes into play tapped. 1 and a mana of any color produces a manland with a creature type, ability and at least a 1/1 at worst. Remove the ability and keep the type would give you a 1/1 creature type for 1. That'd be as good as spawning pool, but spawing pool is terrible.
Factory is a good card, is a 2/2 for 1, has artifact status, has a creature type, and can pump other creatures of it's type. Is Factory broken or unfun, or flavorless? Because I don't see that, and Mutavault is pretty close in power.
Your biggest issue is over the shapeshift ability obviously, which is definitely better in some situations than pumping a worker. But just because faeries gains more power with more faeries doesn't make the cards it uses not fun. What if people like faeries? What if people like Merfolk? Or Goblins? Or Elves, or Shapeshifters? Or Blinkmoths? Or Assembly Workers? The fact that you don't find it fun, doesn't mean others don't.
Bitterblossom is an amazing card, and while I didn't recommend it for my pick, you are biased just because you don't like a deck that used it effectively, and think of the card only as that by association. It's amazing for Sui-black, and a ton of other decks, yes including faeries. I'm sorry you had a tough time when faeries dominated the format you played in, but that shouldn't detract for a card's nomination in whether or not it's fun in casual.