The best LGBTQ character award is chosen by public vote. So the success of Baldur's Gate and popularity of Shadowheart as a romance option probably played a big part. She probably wouldn't have gotten the award if she was a lesser known character from an indie studio. There is an Authentic Representation category, awarded by a panel, which focuses more on the authentic LGBTQ experience.
Ah, my bad then. I still think she shouldn't have been nominated (maybe that was also a public vote?) but if she won by public vote that makes sense. She is a very popular character.
Nomination was also a public vote, then the panel made a shortlist and the public voted from the candidates in the shortlist.
I am assuming the panel made some consideration on the amount of nominations from the public vote as well when selecting their shortlist. Surely Astarion was also among the nominations, but if I was on the panel I would choose Shadowheart over Astarion, because Astarion has already gotten so much attention and is also a more stereotypical sexualized LGBTQ character.
