I think it's not an unreasonable demand but I'd also say that only a minority would do that. I could imagine several reasons for that:
For one, time might be an aspect as others have already mentioned. I'd say that you, personally, also have a different concept of this because you're a creator. You know how much time it takes to draw a comic and you also know how much time it'd take you to make a poll with the pictures. But most of your readers probably don't so to them 15 minutes might feel long.
If I try to look at this from a reader perspective, then 15 minutes are actually quite long. I could probably make lunch in that time or watch half an episode of a series or even do some work-out. Considering that everyone still has other issues in their life, I could imagine that lots of people would rather do something else with these 15 minutes.
The second aspect I could imagine actually ties into this: I don't know if I'm in the minority with that but I seldom read comics for the gorgeous art. I do love gorgeous art - don't get me wrong, I even stop to admire some pages every now and then - but I'm mostly in it for the story.
So if I still remember the story well and don't need to re-read it yet (and honestly, I only reread after a long time or if the comic is one of my favorites), then I'd be unlikely to go back and spend 15 minutes on looking at the art.
Now, if this was a poll about something story-related (e.g. I think I've seen somebody mention easter eggs in another thread a while ago?), then I might be more likely because I'd feel it adds benefit for me. I'd get something out of doing that aka discovering a part of the story I might have previously overlooked. The art? Not so much.
So basically: If I don't get anything out of it, then I'm less likely to spend that 'much' time on it.
The third aspect is a technical one: A lot of users read on the app. Now, I don't know where you live but where I'm from, the signal is often bad and loading a page takes forever. It gets even worse the later the month gets because the provider will reduce the signal (like I'd only have 20mbit of 100 left or so).
So even if I could go through 14 episodes in a quarter of an hour to look for all these designs in theory, it is highly unlikely that I'd be able to actually do so considering these technical issues. I'd very likely spent an hour on this (to reiterate: on a story I already know) so after I'm not even halfway done, I'd probably be totally frustrated and give up even if I wanted to do you the favor of doing your poll.
Overall, I think this is what it boils down to: If I was one of those readers, I'd take part in this poll only to do the creator a favor because I don't get anything out of it myself. Now, I do like doing people favors but only if that doesn't pose a disadvantage to me. So the more time I'd need to 'sacrifice' to do that favor, the less likely I am to do so.