A popcorn popper roaster from my experiences is almost always going to give grassy / underdeveloped beans because you can't really control the temp that well. It roasts them too fast, ie 3 minutes or so.
If you can mod it to stretch out the roast time you might do better. Sometimes playing with the amount of beans you are roasting can help with this.
As far as becoming a great coffee roaster, you have to learn the machine you are using, and don't use cheap crap. ie No you will NOT get good espresso using a whirly bird grinder, no you will NOT get super roasts from a popcorn popper etc. However on that, people who mod them, or use things they have more control of, even the heat gun dog bowl, can get decent roasts.
I can give you a 5000 dollar machine and you can make garbage, or I can give you a 50 dollar machine and you can make great coffee. Learning your machine is the key.
Aaron