It's not just Ice Age 2; it's everything after the first film. Let's start with all of the shit.
It simply shoehorns a romantic relationship in there by magically plopping out a lady mammoth (who thinks she's a motherfucking SQUIRREL of all things, and screw her squirrel family; they sucked so badly that I don't even remember their names) who eventually gets pregnant with a little brat of a mammoth child by the MC whose character was basically, "Oh, I'm a damsel in distress, help me, help me!" As Syndrome (THAT'S how you make an irredeemable human villain, for all of you would-be animated film writers out there: nuanced, suave, intelligent, a genuine threat, and, yes, someone who eats the fucking scenery) would say, "Lame, lame, LAME!"
Oh, and they basically made Diego a side character. If you asked any Ice Age fan who their favorite character was from the first movie, asked everyone who the most nuanced, complex, and overall likeable character was from that movie, it's going to be Diego. I almost forgot he was there in Ice Age 2 because of the stupid shoehorned romance (and Sid, of course, but we'll get to that next), and when there was an actual dilemma in Ice Age 3 (Diego literally starving because, yes, he's a predator amongst prey, and I was shocked they didn't do this sooner, and I was shocked that they waited until the third fucking film to do it), they basically fucked that up as well!
And then...SID. SID THE STUPID, SID THE FUCKING MILLSTONE, SID THE I'M-SHOCKED-HE-LIVED-PAST-BIRTH SLOTH. He was BARELY tolerable (as in the "I still wanted to strangle him with his own entrails" sort of tolerable) in the first Ice Age because he did have some moments where he was actually useful (even when he wasn't even close to being funny). Ice Age 2 fucked ALL OF THAT UP and made him THE BIGGEST "fucking annoyance that I have wanted to throw a goddamned bookshelf through the television, just so I can feel like I actually hit him, or, at the very least, shut him up so I don't have to listen to him" EVER. He nearly killed EVERYONE IN TWO CONSECUTIVE FILMS, and he somehow isn't killed by Diego (or, at the very least, someone who has a lick of sense, oh, wait, nobody has a lick of sense aside from Diego and Manny when he wasn't a fucking simp!) or someone who is utterly sick and tired of his HORSESHIT!
Swear to fate, when my favorite - and funnier, for that matter) animated character named Sid is the villain from Toy Story (although I have an entire rant on how that poor kid was never a villain) instead of one of the supposed heroes, they KNOW they fucked up somewhere.