Jacqui Putnam, who survived the Edgware Road blast in 2005, said people should "think twice" before making light of terrorism.
Defender Smalling, who plays for Manchester United and England, has apologised for his "insensitive" decision to wear the costume.
Smalling's management said he intended to make an elaborate pun on the popular "Jagerbomb" drink by strapping bottles of Jagermeister and Red Bull to his body.
He has been pictured in The Sun appearing to wear an army-style vest with the alcohol bottles with a mock circuit board and cables attached. He also appears to be wearing an Arab kaffiyeh head dress.
Ms Putnam said that even after eight years, people affected are still coming to terms with the terror attack.
"It does not make it any easier when you see people who make light of it," she said.
"It is silliness really. I do not think people who do these things mean any harm but unfortunately the effect it has on people like me and people who are in a worse position - those who lost loved ones - is hurtful.
"I am sure that he did not mean to offend anyone but I wish people would think twice.
"If people could only think twice about the pain this could cause to people who have lost loved ones in these circumstances it would be appreciated by survivors and the bereaved alike."
Graham Foulkes, 61, from Oldham, whose son David died in the London 7/7 suicide bombings told The Sun the 'jagerbomber' costume was "one of the most offensive things I have seen".
Smalling's management company the Wasserman Media Group said in a statement: "Chris and his girlfriend hosted a fancy dress party to celebrate Christmas and their belated birthdays with close friends in the assumed privacy of his own home.
"He dressed in a costume consisting of empty bottles of Jagermeister and cans of Red Bull strapped to his chest in an attempted comedy play on the popular 'Jagerbomb' drink.
"Although he fully accepts in hindsight it was an ill-thought out and insensitive decision, absolutely no harm was intended whatsoever and he apologises for any offence caused."