The average PC without a top-of-the-line GPU is very slow at mining,

Numbers are available. The ratio is GPUs are something like 150x a CPU (going full blast). Botnets are large, but a 150 or 300x difference is hard to make up given how many GPU miners there already are.

(This is not to say that a botnet might not make good money doing normal miners, but it's not really something to worry about.)