One of the problems with large topics is that users may miss the topic subject.
For example, the OP puts a poll in the first post, and after 100 more replies, no one knows there was a poll.
If it is possible for OP to pin the first post of the topic at the top of the screen, I think this problem will be solved …
What do you think?
This is not a feature at this moment in time.
Assuming that all users begin reading from the first post they should know there was a poll.
Yes, you are right. 
But, this is what we expect from the user as a designer. Usually, the needs of users are different, and consider these needs makes programs more user-friendly. 
All users default read position in a topic which is new to them is the OP.
Unless you’re suggesting that users will regularly edit polls into post 1 once the topic has gotten tens of replies this is a non-issue.
Remember, you will only open a topic further down if you have previously read it, or if linked directly to a reply. Discourse tracks the read position separately for every registered user.
If this is important, then edit the poll into the first post, and link to the first post in downstream replies.
The first post will always be “pinned” to the top of the topic 
And there are lots of ways to jump to the top of a topic even if you have a read position in the topic, either from the topic list (try clicking / tapping the post count), or within the topic itself (try clicking / tapping the topic title).
Exactly, users are doing the same thing right now. But they constantly raise this need with us. I wrote it here to be evaluated by Discourse team.
Answering this need is not necessary, but given that it is raised by many users, it can be more effective in UX.
I think because there are a lot of similarities between Discourse and messaging apps, users compare their features.
I’ve been reading every post on this site for about eight years now, and “let me pin a random post inside a topic” does not come up very much. Instead, apply your focus to the natural pinned post which already exists in every topic – the first post at the top.