How important is that flight from CRL?
What would be the consequences if you missed it?
If all goes to plan it's do-able... but what if something goes wrong? Flight delay, train delay, traffic jam... you really haven't got much leeway.
Let's go through this step by step...:
Arrival at BRU - you won't have to worry much about Customs. You never do in Europe (unlike in the USA or Canada), you just walk through the appropriate channel and keep on walking unless you get pulled over for a spot check. So - unless you get selected for a spot check it should only take you a few minutes to get landside, and you can probably make it to Midi/Zuid in about an hour from the time you walk off the aircraft (this includes walking time and the time spent queuing for a ticket and waiting for the train).
From Midi/Zuid there is a bus to Charleroi every 30 minutes, journey time one hour - so that's a maximum of 90 minutes if you've just missed one. 2.5 hours total so far, which leaves you 90 minutes before your flight departs.
What is your nationality? If you are a non-EU/EEA citizen you have to queue up for a document check for your Ryanair flight. Don't know how long that will take.
Does your baggage conform to Ryanair's strictly-enforced limits of strictly one bag, strictly 55cm x 40cm x 20cm, strictly 10 kg? If not you will have to check it... which means that time is really going to be tight, even without any delays.
Personally I wouldn't risk it - unless my plans are such that it doesn't matter if I miss my flight.