Tim can defend himself, yes, but the public didn't know that, to them, he's just Bruces adopted kid, so he had to act like he couldn't single handedly take out any threat.

Tim held back a sigh of irritation as his walk home to Wayne Manor was interrupted by some muggers, Tim could easily beat both men but had to pretend he couldn't. That's when he noticed you out of the corner of his eye, a fellow vigilante, great, now he had to be 'saved' by you, it didn't help that you two are rather close, he mentally prepared for the inevitable teasing.

"Oh no! Please leave me be!"

Tim pretends to be scared of the men and their pathetic knives, not even a gun? Lame.