Should I cancel Lake Como reservations ? Rain the entire time we are there!

No. Today is September 3rd; there is no way that the weather forecast for mid September is accurate that far in advance. Only cancel if you are not losing any money.

If it does rain every day, or most of the time, is it worth going at all?

With it gets to be about less than a week in advance I would cancel if the forecast were for rain.

It is true that weather near mountain ranges is difficult to accurately forecast.
Having lived for many years in the Sierra Nevada mountain range, I can tell you that it’s normal to have afternoon showers in the foothills and mountains during this time of the year. If the forecast has a rain symbol or icon In the graphics for a particular date, you need to look at how much rain is expected and over what period of time.
It’s not unusual to see rain forecast most days near mountains during the summer and fall. ( it’s also not unusual for the forecasts to be wrong and no rain materializes). The most important part of the forecast is: 1. Exactly how much rain is expected. 2. During how many hours is the rain expected to fall.
A day when 2” of rain is forecast to fall over 6 hours is not going to be a great day for the outdoors.
A day when one-tenth of an inch of rain is forecast to fall over just one hour is not a show stopper.
Both days the forecast will display the rainy weather symbol in the graphics and the word “Rain” is applied equally in the forecast for both days. But while the 2” rainy day over 6 hours is a washout— the one-tenth inch of rain during just one hour is just a mild inconvenience.
I would take a look at www.Accuweather.com and drill down on just how much rain is forecast during the days you plan to be there and during what hours before changing any plans.
Accuweather is forecasting some rain at Bellagio on Lake Como between now and September 18 with most of the precipitation falling at night. For those familiar with mountain weather patterns, that is not really unusual. For a bit of rain to fall at night really doesn’t significantly impact plans for the daytime.
During an average September at Lake Como, some rain will fall during 8 days. The only months when there are fewer days of rainfall is December-March.
And those months are freezing!

I live in a small coastal town in Massachusetts, USA. Almost every day in the spring and summer the weather apps show rain. Almost every day in the spring and summer it doesn't rain here. I'll be in Bellagio Sept 9 - 14. It will be the first time since 2019 my wife and I have been able to travel. She has been through some very difficult illnesses the past few years. Trust me, we won't notice the clouds or rain. Go and enjoy it, one never knows what the future holds.

That is so frustrating! We were there last October with forecasts of rain every day, a week out, and we didn't have a drop.

I will say, if you have a lot of rain, the enjoyment of the vistas is greatly reduced and being out and about would not be fun.
There are not a lot of indoor activities there. And Lake Como is an expensive place to just sit inside.

Where are you coming from and going to? Could you double book( cancellable, of course) more nights at one or both of those locations? Or is there another place nearby that would have more museums, cooking classes, or other indoor activities? Then you could watch the weather a little longer and feel a little more confident. It is a magical place but I can understand your concern. Good Luck!