If you were wearing a TF shirt and another person was wearing a TF shirt and there was no acknowledgement at all then that other guy was a jerk, but that's not this situation.

In this instance you say you were "walking around [a shopping centre] wearing a Transformers T-shirt, Transformers hat, Transformers backpack and holding a Transformers toy in my hand"

Perhaps this other person saw a 30ish man in that outfit (and in a normal shopping centre not at a convention or anything) and made the assumption that this man walking around with a child's toy in his hands and saying hello to a total stranger had an intellectual disability of some kind?

I've at times responded politely to people with intellectual disabilities that have suddenly started talking to me on public transport but at other times I've ignored them too. People do that, it's kind of mean but it can also be very uncomfortable to talk to someone with a disability like that.

Wearing a t-shirt is one thing, and it's now a common thing. But Carrying around a child's toy, wearing a backpack and hat goes to a place that no matter how mainstream Transformers is, is not going to look right on someone over 10 years old.