First off, take a look at these two links.
Notice the difference?
1. Look at the profile information:
A (Gender and Age): Tammy's says "Female, 14 years old", while Cartman's (and all the other actual SP characters on here) just say "Male" without the age. On the show, Tammy is 11, and the boys are 10, but since you need to be 13 or older, it doesn't say their age. EDIT: She took out the "14 years old" part after this was made to make this look false, but it was there, so it still counts.
B (Verification): Tammy's (false) southparkstudios.com verification is missing the check symbol that the other SP characters' do. EDIT: She took her fake verification off, so this part isn't there anymore
C (Location): "Colorado" on Tammy's profile is spelled "Calorado". No one else has "Colorado" spelled wrong on theirs. EDIT: She took out the whole location thing so now it doesn't say anything, which makes her still look fake.
D (Link): Tammy did not post the official link to her profile on southparkstudios.com
2. Tammy is just fanning everybody. The other characters don't normally fan you unless you fan them back.
3. I know this is out-of-universe, but on South Park, Tammy was never confirmed as dead or alive, assuming that she had Syphillis, as she gave Kenny it and it killed him. So we have no idea whether or not Tammy should really be here.
4. Look at who's a fan of her. None of the other South Park characters are. That's because they know she's not really Tammy.
5. Another out-of-universe thing: As you may have noticed, only main and major characters are appearing on Fanpop. With Tammy only appearing in one episode, why would they put her on without putting on various other minor characters?
Wondering why these are so? It's because she's just a fangirl pretending to be Tammy
And, if you see her, drop her as a fan, and report that impostor.