I guess it depends on how you define "good".
A good gambler for a casino is someone that puts all his life savings in the casino as fast as possible.
A good gambler for a person looking to earn money is someone who wins big once and stops gambling forever after that.
A good gambler for someone that wants to be entertained in the casino is someone that doesn't spend more than his budget every time and he always have fun while doing so.
Etc.
Yes. You equally have a brilliant point listed above, and I agree with you that the definition of a good gambler truly varies from one individual to the other, because just as to the casino, a good gambler is someone who gamblers all his life savings in the casino.
To a family member, a good gambler is someone who wins big, come home with all the money for merriment. While, To a friend, a good gambler is someone who wins big, takes friends to the club and lavish money on drinks and women.
While to the Uber driver, a good gambler is someone who gives an extra tips everytime for a ride.
But in summary, a good gambler is someone who is able to manage his time and money efficiently that he has both time/money for family/friends, work and his gambling activities