Immoral by whose standards? Are you asking because you feel an honest person would make a living by selling/creating goods and services? Poker is a zero sum game (less technically when you consider the rake?). The casinos
or websites are providing a service and take their cut for it, the players aren't doing anything productive except earning money or having fun. When a poker player makes 50K in a year its not because anything useful was done for society its because he or she took the money from other people albeit in a series of voluntary contests. Is that what you mean by immoral?
If you spend your whole life playing poker, one could argue that you contributed no real value to the world, you just took money that was probably earned by someone at a job. If you happen to think you are your brother's keeper and owe some debt to society then perhaps poker is immoral. On the other hand, maybe you just want to play a game and have all these self righteous idealists leave you alone :P It should be up to you to decide, I'm sure most people here would agree with me when I say it is not immoral