"That girl is cute, isn't she?"
To me it genuinely feels like Duolingo is trying to make me slowly realise that there other correct ways to say something. I also think that even though it not always consistent (like it won't let you skip the は usually) it is somewhat justified, because it can't really tell whether you forgot the particle or are willfully skipping it. All in all it is a nuisance I'm fine with.
Tbh if the は is there, use it. If not, it's clearly not necessary. It's not difficult in the least.
I like that it shows us the various ways to form sentences, because...in the real world...Japanese people aren't going to change their dialect to please a simple mind, as if they'd know your knowledge is vastly limited in the first place. There are many different ways of communicating within every individual language, and the point here is to learn another language. I wish there were MORE Japanese duolingo lessons.
Besides, if I'm not challenged, I'm bored. I don't play video games because they're easy...I play for the worthy adversaries.