Maybe it is a regional thing. In my experience here Mixed is very competitive. I play 8.0 and 9.0, and I've never heard of anyone taking it easy on the girl in matches. In fact, it is completely expected to hit it as hard as you can at the girl and to target the girl. My team and partners would not be happy with me if I was taking it easy on a 3.5 girl, and I probably wouldn't be put into matches if I did that. The 4.5s with the huge serves will absolutely take it to a 3.5 girl, and they will be the target of every overhead. The 3.5 girls that are playing 8.0 Mixed are playing because they can handle those shots and can often return those serves often enough that it is not an automatic hold if the 4.5 is serving. I'm at somewhat of a disadvantage in Mixed because I don't have that huge fast serve or huge kicker, but I can usually make up for it in that if I hit it to a 3.5 or 4.0 girl, I hit it well enough and I am fast enough that they usually can't direct the ball away from me.
But I have been nailed in a match, my 3.5 female partners have been nailed multiples times, and I have hit women and men with many overheads, it is just a part of Mixed, and I have never seen any hard feelings shown from hard shots directed at the women. And I have never seen any injury from a tennis ball, as all these shots are mostly at the lower body.
Social matches are a different story, of course, you should use your own discretion.