The reason is by definition, doormat is something which everyone uses for wiping their dirt. But in Tanisha case, its not. She gives back to anyone who is against her.
Whoever calls her doormat, tell me one situation when she is being used by doormat by Gauhar Khan or Sangy, Andy or anyone else for that matter.
She is like that only when its a case of Armaan.. so yes, for Armaan she is in blind love with him and she stands like that which could be because of her care towards him and she knows very well that if she reacts, it will be damaging to her relation which she does not want (and thats what blind love is)
The only lady which portrayed herself as doormat is "Sherlyn Chopra" from BB4, wherein everyone used to get their work done through her but NOT Tanisha
Just to give one example, if a mother is very caring and listen to all abuses from his son... does not make a mother doormat.. its her blond love towards son which makes her doing so. By saying so, I am not saying that Tanisha is acting like a mother of Armaan but I am just giving an example in this situation.