Can you tell the dog owners to bath their dogs before bringing them?

Sure! You can make whatever requests you want of your clients (ideally reasonable ones, lest you scare them off :P). Seriously, though, I don't think that requesting a dog be recently bathed is a particularly unreasonable requirement, especially if your clients' dogs spend a lot of time outdoors. I would be careful with how you word it, since you don't want to come off sounding like you're accusing them (or their dogs) of being unhygienic, but if you approach it tactfully, I think the risk of offending is pretty low. Everyone knows dogs don't smell like roses, and it's totally understandable that with many dogs coming through your residence you're going to be far more likely to notice the cumulative smells of all your guests and have a harder time than your average dog owner keeping everything smelling nice.

You may also consider bathing as a service - if a client is unable or unwilling to bathe the dog prior to drop off, offer to add a bathing fee to their stay so their dog won't dog up your home, and will come home smelling fresh. You may even consider drawing up a written agreement that includes your bathing requirements (along with any other requirements you have) that the client will sign before you book the stay, giving clients the choice to bathe their dogs before drop off, or add on the service for $X amount, and explaining that if a dog arrives unbathed and they haven't already paid you for the bathing service, an extra fee will be added to cover it.