I am now writing my thesis and I am thinking about if it useful to make UML classes diagram for Android App which have about 35 classes. If i make this diagram it will be very big and confusing. I know about plugins which makes this for you but I just thinking it is good for something?
Well your app probably has some data structure that should be documented at least. One big diagram would be unreadable and unclear, but a smaller diagram for each part of your structure might be still relevant.
Of course if the details of your implementation is irrelevant don't do an UML.