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It definitely should be able to handle that as long as you program it well and aren't doing many resource incentive things at once with it. Speaking from experience, the pi is able to handle multiple small bots very well(though it might hiccup if you try and start them all at the same time). If you have only that one bot running, you shouldn't be in a place too bad.
All you would have to do would be to stream the blockchain(easy to do with dsteem if you choose to use nodejs to develop your bot), check all operations to see if the type was comment, and then see if if it was a root post or a comment on another post, and finally check to see if the body contains the words you are looking for. That shouldn't be too hard for the raspberry pi to do as I've done some harder computations and it managed to handle it well.
If you encounter a particularly large post, I don't know how well it might fend against that with the limited 1 GB of ram, but I think it should be good. If it does end up crashing because its using up too much resources, you could choose to rent a server from a service like 