You'd have to really work at it. If you're just playing it for fun or stress relief, no, you're probably not challenging your brain enough. If you're really working on the strategy and increasing your score, then yes, it could be giving you the mental exercise you need.
It's like of like going for a walk. If you're just strolling along, enjoying the day, it's great, but it's not really "exercise" of the kind you need to keep your heart healthy and your muscles strong.
Nothing wrong with having an enjoyable hobby though!