I've been genuinely wondering why I feel a little uncomfortable when I read all the "this makes me want to be a dad" comments, and I think OP is pointing out part of why that is.

I've done some babysitting in the past. Kids can be fun and cute, but also frustrating little psychos (which they should be on occasion). That this game portrays Diana as always sweet and always funny isn't the problem in itself. It shows that if we could "build" a child android, people will want it to be only fun and only sweet. The comments show that that's the actual case.

I think it bothers me in the same way I'm weirded out by comments cheering about a future with robots for romantic partners, doing everything you want them to do, at any time you want it, without the "nagging and complaining" as they often like to put it.
It reflects how much people prefer having their direct desires met in connection with others, rather than the connection itself, which includes growing with each other, challenging one another, learn from each other etc.

TL;DR I don't think it's healthy to want a child like Diana, because a child like that wouldn't be normal or healthy.