Assuming you're looking at the old b&w series (I didn't get round to watching the remake they did sometime in the last decade), then I'd say it can be tragic at times, but the overall feel of the series is bittersweet rather than maudlin. A perennial theme is the generational aspect: some things change, some things don't, tragedy and happiness co-exist in families and ebb and flow over time.
If you want a "happier" series, go for one of the other series I mentioned instead. House of Elliot has a lighter feel (though obviously like any drama it some sad events within it), partly because it's set in the roaring 20s and the business the sisters work in is naturally more cheery. I really liked it a lot.