No one can tell you if you're a good rider or not without seeing you ride. As a beginner, of course you're not a 'great' rider, but you may be a 'good beginner'.
If you have a good, solid foundation in your riding - quiet hands, a solid leg and stable, balanced seat and upper body, then yes, you are a good beginner. If you're flopping around all over the place, with legs swinging, hanging off the horses mouth of pulling back on the mouth all the time, then you're probably quite a 'raw' beginner and would benefit from some lessons on the lunge with no reins and stirrups to improve your core stability.
We all fall off, I would say that you falling off when the horse picked up canter probably indicates that you don't have a very balanced/solid seat at this stage. I would go back to really concentrating on your seat, balance and position in general for a while before you have another go at cantering.