I'm not quite sure why people are disappointed with this film. What did they expect from it? The blurb couldn't make it more clear that it is a ridiculously cheesy spoof: "When a meteor smashes into a local museum, the fury of a partially frozen, 40,000 year old mammoth is unleashed on a small country town". It goes on to mention that it has to be killed before its "rampage of death and destruction gets to the outside world".
Did it not strike the critics as strange that one mammoth would be any more difficult to control than an elephant?
The film is full of classic B movie ingredients: aliens, government officals, scientists, 17 hours to save the world, cheesy script, etc etc. But, whereas some spoofs can actually be good films, operating as observations on the genre of spoofs (Hot Fuzz, Sean of the Dead), this one remains well and truly lame. The special effects are bad, the plot feels like it was invented as they filmed it, and the obligatory American sentimental scenes go on far too long. The alien premise seems fundamental to the story, but seems to get forgotten immediately, before being tacked on again.
The humour sparkles occasionally, but really not enough - it would have been better if they had gone the whole way and made it into a comedy (hiring a script writer might have blown the budget, though).