I made this puzzle.
Everything went relatively smoothly at first, but then I got pointed out that I needed to use mathematics with geometry. Also, my puzzle got a lot more downvotes than usual. Is it because I forgot a tag or is there something else that is bad?
(No conflict of interest - I did not vote either way on the question being discussed here.)
I am 100% sure this will not be because you 'forgot a tag' - retagging is a very common activity here on Puzzling, and is usually done by more experienced users who realise another appropriate tag applies when the original poster may not have noticed. That's not to say it only happens to relatively new users - even experienced users forget tags; it happens to everyone. For that reason you are very unlikely to have been penalised for just omitting an appropriate tag.
What is more likely, though, is that when other users saw the accepted answer, they felt this was more of a 'trick' than a true 'puzzle'. After all, the method upon which your solution relies could apply to literally any character, rather than just the number '2', plus it depends on assigning a pretty solid width to a two-dimensional object, which many users probably didn't consider a very satisfying solution.
Ultimately, it's a lateral thinking question whose answer nobody could reasonably logically deduce, and instead it relies on a line of thinking that doesn't really fit with the geometry of how two-dimensional shapes even work. Plus, what seems like a simple teatime brainteaser (thereby enticing people into trying to solve it) ends up having an answer that seems more to be a joke answer than a true solution, which probably miffed a lot of the folks who'd been putting in the brain effort to solve it.
At the end of the day, I cannot speak for everybody - different people will have downvoted for their own personal reasons - but these are some observations based on past behaviour I've seen in this community and my own understanding of what tends to make a satisfying puzzle. Hopefully, some of what I've said here might prove helpful as you go on to design other puzzles in future...
