I would need to go back and rewatch the early episodes to have a confident take on Mr/Ms Garrison, but the biggest problem with the character IMO is that they are consistently one of the most garbage characters in the series, yet for a long time were the only part of the main-ish cast who were gay, and afaik is the only trans character at all (I'm blanking on any other trans characters besides "Heather," a fake-trans, although I haven't watched the most recent seasons). So while I would argue that Garrison was portrayed as a bad person who happens to be gay/trans rather than bad because of their gay/transness (basically the whole point of the episode where the kids are uncomfortable with Mr Slave, which the parents misinterpret as homophobia), it's not a great look for that to be the only LGBTQ+ character outside of a few minor characters, and I certainly would be understanding of folks who take offense at this. But as you say, it's not all black and white; even Garrison's sexuality/identity journey is handled sympathetically at times, and the other LGBTQ+ characters are handled quite sympathetically on the whole. That said, the series leans very heavily on the "G" part of that acronym, but that's not too surprising in a show that's always been from a primarily male perspective.