A mutual fund although generally quite safe is still investing in the stock market. Is that predictable? Not really! Is it safer than doing it yourself? Probably. Reward is alway going to be proportional to risk and if you want larger returns that the 5% or so a bond or CD is going to give you, well, you have to take a chance.
To answer the question directly I would say yes, as long as you are in it long term. Over all the market will increase faster than any other means of investment. If your thinking more short term I would stay away!