I'm not saying whether or not Mach Rider is a women, I'm just noting that the trophy isn't a valid argument.
Also, while the first looks very different from the second, it's not like Nintendo hasn't done similarly strange things before. Samus' power suit is notoriously broad shouldered, to the point that neither a woman
nor a man could fit inside it without doing serious damage to themselves. This makes her look much more masculine than her actual body type.
I mean, it's more connected than your statement makes it seem. Each round played slowly revealed a different part of the image, until eventually the final one was shown.
"Each time the player completed a level, an image was gradually revealed of a woman with a dagger standing next to Mach Rider's bike. It is unclear whether this is meant to represent Mach Rider's true identity, or if she is a survivor in the gated community of the resistance, as the only plot text in the game describes the 10 mission scenarios and she is not present in the NES original."
It's ambiguous. Probably on purpose.