CMC is now in progress of improving their website and make the CMC ranking by liquidated exchanges. So, it won't count only the volume which can faked by the exchanges itself. I'm sure it's not the CMC fault but it's the market which manipulate their volume by trading bots or something illegal like that.
Sure you cannot blame everything on CMC itself for all the fake volumes but they are helping in adding fake and scam coins in the market by accepting money and people tend to fall for that, there is no clarity regarding the manipulation in volume and whether they are aiding them but now they will not take the reported volume alone and there are companies that are monitoring the exchange traffic and with time we will not see these fake volumes from exchanges.