Yes, it's fair to downvote a post — period, full stop. Anyone can downvote for whatever reason they like.
Why are “wrong” votes allowed though?
It's a necessary feature of the site's functioning. To make the site work, we need accurate scores beside posts. To get accurate scores for posts, we need as much data as possible. To get that, the site's design and software encourages as much voting as possible of all kinds, because more votes means more accurate scores.
A consequence is that “wrong” votes (both up and down) must be allowed, because any barrier to voting would drastically decrease voting.
Fortunately, by encouraging many votes, the effect of “wrong” votes is automatically fixed: the more votes you have, the more “wrong” votes disappear into the statistical noise. What matters is what most of the votes say, not what one vote says.
So votes being at the full discretion of the voter is a necessary, non-negotiable feature of the site since, without it, the site would be really bad at the job it exists to do. It's a bug, not a feature. You get to vote however you like — so does everyone else. Enjoy the privilege, and accept that it means everyone else will too.
But still, why is the question “low effort”?
Although the basic fact is that the votes don't need to be justified at all, we can still look at why this question could be judged “low effort”.
Here is your objection, rephrased: But if it would take a lot of effort to answer a question, how can it be low effort to ask it?
Notice the difference: high effort to answer; low effort to ask. The fact that these apply to different things — the answering and asking — is the key to understanding why a question can require high effort to answer but still count as unacceptably low effort to ask.
In this case, “why is this low effort” is easily answered. The trouble with this question is that it already contains its solution, and the solution takes effort to use. The real problem you have is one that this site is not made to solve: that you just don't want to expend the effort to use the solution you already have.
That's low effort. “I know how to get the answer to my own question, but I don't want to do the work. Here is the work that needs doing; please do it for me so that I don't have to waste my own time,” is going to get downvoted for low effort.
Answering the question might be high effort, but that's not what the downvote means — the downvote is on the question. It means that the asker has not made enough effort to solve their own problem, or is asking the question to try to avoid making any effort to solve their own problem.
Asking other people to do work that you can do yourself (but don't want to) is making low effort.
More generally stated: how much effort it takes to answer a question doesn't matter; what matters is whether the question appears to exist to help the asker avoid doing work. When it does, it will naturally attract a few or many downvotes for low effort.
This is statistical though, not absolute
But keep in mind that voting is purposely statistical — what one voter thinks is low effort is not important, it's what the hypothetical “average” voter thinks that the site's voting mechanics are designed to harvest. So if some voters think a question is low effort, it will get some downvotes, and if many voters think it is low effort, it will get many downvotes. If there are many voters who disagree, these will be counterbalanced by upvotes. The collective judgement is what matters.
Overall the point is this: one downvote is nearly nothing. If you want to understand why someone would vote a question for low effort, the above is relevant, but it's really a waste of time caring about one downvote. It's only when more or many votes agree that it becomes worthwhile to try to parse what they mean.
(And this is why SE doesn't/won't ever require commenting when downvoting: it would wrongly focus on individual reasons for voting, when SE's voting system is designed to focus on an aggregate judgement, not individual judgements.)
