In my opinion: yes and no.
Summarising the key question in a post is always recommended and while we encourage the OP to include it or make the question concise/readable enough that a summary isn't required, we do get plenty of long posts that can be rambling in nature.
One of the most common edits I make and probably the second most common after typo fixes on the site in general is to make questions more readable. Very often that means identifying or adding the core question.
What I would not recommend is to add that summary with an actual "tl;dr". Given the literal meaning it can still be taken the wrong way but more importantly there's just no reason to include it. If you add a single sentence as its own paragraph at the top or end of a post that will automatically add emphasis and be taken as the summary or key question. If you want, you can emphasize further with bold formatting or by adding a horizontal rule before the rest of the post. In addition, while it's pretty common netspeak at this point, there are still people who would be confused by the term, especially those not familiar with English language internet slang.
So tl;dr: summarize or emphasize the core question rather than adding actual "tl;dr" blocks.