Is it OK to ask my host for an externally organized stay which hotel is being booked?
Given that it's in China (and I'm assuming you're not Chinese), I would let the host do it for three reasons:


It's important to be gracious to the host. This is a general rule in most cultures, but particularly in East Asian ones. Letting your host be a good host is part of this. Trust your host's judgement here.
My experience with Chinese hotels from Shanghai to Xinjiang is that there is minimal to negative correlation of the quality of the hotel to the website or the official star rating.  It is highly doubtful that you could ascertain anything superior from afar than what the local person would know. 
Logistics. The host may want to put all of the people in a particular hotel (or spread them between a particular few) because of logistical reasons: geography and   they have only one van to pick everyone up, etc. etc. 


I would let your Chinese hosts handle everything. If you have particular needs (room must face towards the south; hotel restaurant must have halal food; etc.) then let them know. Otherwise, let your host be your host.