If you really wanna throw a twist in, only mention the sexuality and/or gender of all your cis-het characters!
But in all seriousness, I do believe it's important. Obviously you should write characters first, but I think that if you can find a way to work it in, do it. If you're not comfortable being explicit, be subtle! Not too subtle because people will find any excuse to argue why their favorite characters can't be LGBT because reasons, but you can add little characterizations like having them briefly flirt with a same-sex character, having them complain about their binder being too tight today, or them trying to remember if they told so-and-so at work about their new pronouns, and etc. Be creative! Being inclusive takes effort, and if you think it'll bog down your story, then I'm sorry to say that you're just not thinking very creatively. All it really takes is a couple of throwaway lines every few chapters or so to tell (and remind) your readers that a person is not straight or cis.
Now, here's an example that I cobbled together in a few minutes:
"She's pretty cute", said Api, referring to a slim, pale young woman with pink lips and cute hips. "Think I have a chance?"
"In your dreams," said Lyra. She grinned at her friend and patted his chest. "Watch this."
Lyra got the woman's attention with a prolonged wave. As soon as the woman looked up, Lyra made dreadfully loud kissing noises, suggestively licked her lips, and winked, causing the poor stranger to blush bright pink and sharply turn away. Api shook his head as his friend sulked.
"Pro-tip," he said. "If it wouldn't work for me, it damn well won't work for you."
That wasn't the best thing that I could come up with, but I think it's pretty obvious to say that Lyra is, at the very least, not-straight, and I didn't even have to directly state it. But that example is pretty blatant, so let's go with something more subtle:
"Hey there! What's a lovely lady like you doing in a place like this?" said a lanky man that was loitering in the entrance. Paprika almost visibly cringed.
"Me? Uh, nothing, I guess. Just gonna go get a drink," they said non-noncommittally.
From that, I think it's not too hard to guess from Paprika's reaction to what the man said, as well as the pronoun I used, that Paprika most likely does not identify as a "lady", even if they may look like they do. Those are just a couple of examples, and whether or not you think they are any good, I think they get my point across. You don't have to have your characters or the narrator halt the story, look at the audience, and say "I am/This is so-and-so, and I/they am/are (insert LGBT identity here)." It's possible to have fun with it, mix it up, and stretch your creative legs. As we like to say around the writing sphere: show, don't tell. If you're not sure that you're getting someone's characterization right, or you're worried about coming off as offensive, ask someone in the know. There are thousands of fellow writers out there who both know better and are willing to lend a helping hand, some of whom are on this very site.
So there you have it. TL;DR, absolutely you should add LGBT characters and identify them as such, and there are multitudes of possible ways to do it. Thank you for coming to my TED talk!