In a context like this:
The mobile responsiveness of this app is horrible!
And a related example like this:
Let's make this entire mobile app responsive
How can we translate the word "responsiveness" by itself into Brazilian Portuguese?
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The adjective responsivo is being used to refer to responsive UI. We can separate "responsive" from "adaptive", so, despite it not being a very common word, it will be immediately associated with responsive layouts in applications.
responsivo (masculine) – layouts
responsiva (feminine) – applications
I think Interactive fits good to it too. "Interativo" in portuguese (BR), because the direct translation of "responsiveness" is "responsividade" which is not very used here, it still understandable but it is never used.
When we think about response we think also to interactiveness, to "talk" to the user, communicate with them. I think "Interativo" fits to your question.
"The mobile responsiveness of this app is horrible!"
"A Interatividade desse celular é terrível!"
"Let's make this entire mobile app responsive"
"vamos fazer esse app mais interativo!"
Hope it helps