The question, when a proportion of the population commit a proportion of a crime, is whether the sizes of the two proportions match. This is difficult to gauge when there are very few data points.
When it comes to US trans mass shooters, there is insufficient data to make any judgement about proportionality because of the rarity of mass shootings compared to, say, ordinary murder, the rarity of trans people committing these attacks and the low number of trans people in the population.
When it comes to US male mass shooters, the proportion of shooters who are male is far far higher than you'd expect from a sex class that make 50% of the population.
The point of the thread wasn't to cast shade on trans people, but to express dismay that the police and press won't tell the truth about what sex a violent offender is any more. When Myra Hindley was arrested, no one doubted the reports that a woman had jointly murdered multiple children. People were horrified, it might have taken a few minutes for what they were reading to sink in, but they didn't question the reported sex of the offender. Same for Rose West, because until the last decade or so, reporting was by sex. Now, we find ourselves thinking "they say it's a woman, but is it another instance of a man being called 'woman' like Isla Bryson etc?" because it's statistically unlikely that a woman would commit such a crime and we know that the press and police will lie to us.
It's not about trans people, it's about truthful reporting.