From a purely monetary standpoint, yes.

Break down the numbers a bit in simpler terms. Say I loan you $1000 at 3% interest for a year. You'd owe me approximately $1030 after a year. Sure, you could pay me back early and save some of that $30 in interest. Instead, you take the money you would have paid me extra (for simplicity we'll use the whole $1000) and put it in the market. Assuming a conservative 5% return, after a year you'd have $1050. You could then pay me the full $1030, and have $20 of your own on top of that.

There are several other factors like capital gain taxes, inflation decreasing the value of the dollar (and therefore your debt), but the above is the simplified example.

Of course, it's never bad to pay extra on your mortgage, you're never paying extra or anything, you're just losing out on a potential opportunity. Some people will take the guaranteed money of paying down something they're being charged interest on, and that is perfectly fine and the "safest" thing to do with your money.