You've raised a good point. However, I think that the better audience reaction is probably in reflection of your own change in character due to the introduction of the puppy, rather than the puppy itself being the focus of attention (or affection in this case).
I believe that props are only dressing for effects - the fact that they are balls or bunnies is irrelevant. The bunnies don't really look look like bunnies do they, but what does seem real is the performer's change in mood and behaviour towards the sponge. The performer's reaction to the props makes them seem real to the spectators, it's not the props themselves.
It is true that cards
are uninspiring. The values of cards mean very little to spectators. They know there are 52 of them and they are mixed up and that is all that matters in terms of cards. It's not untill you
relate an object to the spectator that they become interested. You tell them it's a cute dog, get them to sign a card or whatever - either way it's connecting with them on a personal level. Relating a regular object to their ideas about cute dogs or putting a signature on a card gives the feeling of ownership and familiarity over the object.
I think this is what makes most good prop tricks powerful.
I think it's really about familiarity. You know when comedians do that think when they make a medioker joke but later they recal that joke and the laugh is huge. It was as if the audience was made familiar with something, then the comedian seems to connect with them on a very personal level with that thing. It seems funnier because they feel attached to the idea initially proposed. They are familiar with it so it's funny.
Sorry I rambled but I think it could probably make sense
