Just using the generic term ornament can mean many different things. The little acorns and bird houses that many make should not have any issues with weight.
Other styles of ornaments, depending on their size can start to tip the scales. Keeping them under 2 oz is a good rule of thumb. I have weighed some of those resin type, from the store with the gold crown, and they can be over 4 oz.
These pictured are all 2.75" x 7.5" and just under 2 oz, Could I have made them lighter, sure, but I don't want them so fragile that they break the first time they fall off the tree. IMO an ornament should be able to put up with a little punishment and last for many holiday seasons.
If an ornament weighs too much, put in a place of honor in it's own stand.