Pay attention and really think about how people make buying and selling decisions and about the economy as a whole, then yes you could make a lot of money.
BUT always leave enough money so that if you lost all of your stock you'd still have enough money that everything would still be OK.
Investing in the stock market is a good decision because it is a place to earn more on an investment than in traditional ways, such as a bank account. It is not for everyone and should be carefully considered. However, I do believe it is a good decision and can be successful for many.
For the most part, all of us need to generate returns beyond what can be had outside of the market. With rates so low, most people can't just afford to use term deposits and bank accounts, and need to use the stock market to grow their savings. Historically, market returns have helped build wealth for those who are patient.
Investing in the stock market is a good thing to do with some of your leftover money. This is because you're both contributing to the growth of business in the country, and also earning some extra money for yourself, at commonly a higher yield than a regular savings account. Many retirement accounts are based off stocks, as well, and it's a good idea to invest in some aspects of the market when you have say in where your money goes.
No, investing in the stock market is not a good decision. Very few people actually make decent money off the stock market, and the majority of the investors end up losing their life savings due to not understanding how the stock market truly works. People who want to invest should find safer alternatives, and not try to become rich quickly which generally results in becoming broke much quicker.