The only experience I've had with non-musical buskers was at the single renaissance fair I've participated in. The jugglers, fire-eaters, and magicians that performed there were much more blatant about soliciting tips than I've ever seen a busking musician be. It seems to be an extension of the persona that they develop as performers. When performing, they are in character, and the charcter they are portraying is often a loud-mouthed, wise-cracking jokester. Being pushy about tips is just part of staying in character.

Fortunately, blatant solicitation on the part of street mimes would mean stepping totally out of character which should, hopefully, result in their eventual starvation.