Fair comment is a defense against defamation claims where the statement at issue is an expression of opinion, reasonably based on (preferably disclosed or well-known) true facts, which is on a matter of public concern, and which is the honest opinion of the speaker which was not made specifically to cause harm. The basic idea is that on an issue of public concern, an honest opinion isn't defamatory unless it makes a reasonable listener think the speaker knows something that is in fact not true (the classic example is "In my opinion, Jones is a liar:" here the speaker implies he knows that Jones lied, which is a statement of fact that can be true or false).
In your example, there are questions that have to be reached before fair comment even enters into it. First off, in US law any defamation claim has to be for a statement that a reasonable person would interpret as being a statement of fact or (if it's an opinion) implying a statement of fact (like "I think Jones is a liar"). This is context-dependent. "I wouldn't let my child be alone with someone like famous convicted pedophile 1, famous convicted pedophile 2, or politician" is more likely to be read as "politician is a pedophile." A marked work of fiction is unlikely to be read as a statement of fact. If the statement is read as a simple opinion, or as pure fiction, there's no defamation under US law.
Once we've established that an express or implied statement of fact is at issue, fair comment asks whether the fact is true or false and whether it's an issue of public concern. But in the US, with a politician, you'd never go for the fair comment defense, because you have a much stronger defense -- a politician can only win the suit if they show that not only is the statement false, but the speaker actually knew or strongly suspected that it was false. It's not enough that a reasonable person would suspect it's false; it has to reach the level of recklessness, which is extremely hard to show (public figures in the US do not often win defamation suits). This is strictly easier than for fair comment.
If the politician wasn't a politician, and was actually a private figure, then fair comment could enter into it. In that case, the question is whether the statement is a reasonable opinion drawn from the (true) facts that lots of people didn't want their daughters alone with the person. While it's context-dependent, that could very well be fair comment.
