You forgot to add the '
in my opinion' at the end of that sentence.
Now it's not clear to me that Tim Farron's comment were about Brexit: There are other issues (such as law and order) on which the LibDems view Tory policies as damaging to our open society, so it's very plausible that he was equally referring to that. But let's run with your assumption that it was all about Brexit: Sure, you personally believe that Brexit needlessly damaged the UK. That is your
opinion. But there are many other people (including myself) who are not of that opinion. Therefore Tim Farron's comments are controversial: Not everyone would agree with them! Of course, I'll still totally defend his right to say them, because we are a democracy, and hopefully some of us are able to distinguish fact from opinion!
To my mind, the issue is that you and
@nw1 are showing double standards in your posts: You're objecting to a Conservative expressing his opinions in his resignation statement, while supporting a LibDem doing exactly the same thing!
I have not up to now said anything, critical or otherwise, about Smith's comments. I simply pointed out that he has a right and it's fine for him to say them in his resignation speech.
But for the record, here are is the relevant part of
Henry Smith's resignation statement. I invite you to read it and observe that there is nothing childish about it: There's certainly stuff that I can imagine you and others would disagree with (there are actually one or two bits that I might disagree with too), but there's nothing childish (unless you somehow imagine that anything that you disagree with must automatically be childish

). He is simply stating his opinions, as is normal in a democracy!