My goodness. I'm not a big Pixar fan, but I can see why they're generally considered the pinnacle of US CG animation: the competition is simply that much worse. Monsters vs. Aliens is supposed to be a comedy, but the jokes are so painfully bad that it feels disrespectful to other comedies to even call it that.
These films also don't age very well. Monsters vs. Aliens is a decade old and looks cheap. Ugly character designs, poor detail and generic camera work make for an unappealing-looking film. That the problem with films that hope to get by on technical chops alone, you really need aesthetics if you want to last.
But it's the atrocious and desperate attempts to be funny that completely ruin this film. The comedy is mostly just loud. The jokes are very predictable and the running gags are annoying, adding insult to injury. 90 minutes doesn't sound very long, but when every single minute of it is painful to watch, it's pretty damn excruciating.