Only the object of the postposition needs to be in the oblique case.
The postposition in this case is में (in). To see what is the object of the postposition, look for the part of the sentence that answers the question किस में? (in what?).
In this sentence, the answer would be 'कमरे' (room). So, only that is in the oblique case.
If the sentence were instead 'She is in that room', it would be 'वह उस कमरे में है' because the object of the postposition is now उस कमरे (that room) and both these words should be in the oblique case.
Oh no I mean why can not we omit the in the English translation. I mean is she's in room can also be a valid translation? Thanks for your concern on my previous doubt. And for your idea, I'm not able to find notifications in duo website. I'm getting the notifications from my mail and thus the late reply.
English has both indefinite articles (eg: a, an) and a definite article (the) and one or the other is used to specify definiteness of a common noun (so, 'she's in room' won't be grammatically correct). In contrast, Hindi does not have a definite article and so, the absence of an article may indicate definiteness. The word for 'one', एक serves as an indefinite article (though it is often dropped and definiteness has to be inferred from context). Therefore, वह कमरे में है। would be "He/She/It is in the room" and वह एक कमरे में है। would be "He/She/It is in a room".
Unfortunately, I don't think Duo has website notifications.
It's not plural.
The thing is that in Hindi, whenever a word is the object of a postposition like में (which are equivalent to prepositions like 'in' in English), they take a slightly different form. It is then said to be in the oblique case.
The oblique-case forms of many singular words are exactly the same as the plural forms but they should not be considered plurals.