The only appropriate and safe answer is "Yes".
Here's why. No matter what the problem is, there's only going to be a couple of questions which will help you get closer to the solution. All the other ones are going to be stupid questions. And let's face it; no halfway sensible person asks questions they already know the answer to, or which they could figure out the answer to by sitting quietly for a second. So if you ask a question, it's because you don't know stuff, and the chances are that you don't even know enough about the subject to know what a good question would be. So the chances are that any question you ever get asked, on its merits, is going to sound pretty stupid. For that matter, the chances are that any question you ever ask is going to be stupid.
If you're unlucky enough to work in a job with easily defined parameters, most of the questions you're going to get asked will be non stupid ones, but they'll all be the same question, more or less; they'll be versions of "can you tell me when the next train leaves/whether this item is in stock/what price extra fries are?". If you're lucky enough to have a job that pays well, it's almost certainly paying you well because it requires you to think. And if you're paid money to think, it's invariably because there are lots of people coming by your shop who find it too much trouble to think about the things that you're being paid to think about. Those people are going to ask you stupid questions.
Rejoice. For two reasons. The first is that your job wouldn't exist if it weren't for stupidity. I've made a good living for twenty five years out of the fact that the world is full of people who don't know how to do things that I know how to do. It's frustrating dealing with the consequences of stupidity, but whenever I find myself wishing that for once people would just get a clue, I remind myself that if they ever did, the first sensible follow-up would be to put me on a farm somewhere. And the second reason to rejoice is that stupidity is neither general nor incurable. There are irretrievably stupid people out there, but the majority of stupid people are specialists. Just as no-one can know everything, no-one can be ignorant of everything. And people can, and sometimes even do, learn.
So when someone comes up and says "Can I ask you a stupid question?" fight the temptation to embroider your reply. It would be amusing to say "Apparently, you can." or "Only time will tell." Lord knows, I've said that often enough. It's tempting to say condescendingly "No such thing as a stupid question.", although I hope I've just talked you out of that fallacy. The simple and true answer is "Yes." With no baggage or condescension, and with the humble awareness that your own turn in the barrel is coming, say "Yes." It won't make you feel much better about the idiocy to come or the mental gymnastics needed to jump over it. But at least it won't make the person asking the question feel like any more of an idiot than they have to.