OK, referencing the suggestion to observe the crowds and what they are wearing on some webcams, here are a few to get you started:
https://www.skylinewebcams.com/en/webcam/italia/veneto/venezia.html
You can see attire a little more clearly on the Rialto Bridge cam, the one of the walkway beside the canal.
Funny, for staying cooler, it seems to matter more what I'm wearing on top than on the bottom so jeans - and mine are fairly light weight - don't bother me, although I know that's not true for everyone. While we sink wash/drip dry most of our shirts, we hit a laundromat every 1 and 1/2 weeks or so; that's when the jeans get a wash-up/machine dry. As you asked, Anne, I'm sure you know that denim, skinny or otherwise, can be dressed up or down with a scarf, a dressier top, etc. if you already do so at home. Spiffing up with easy-to-pack touches versus, say, entirely different outfits, helps reduce the bulk of stuff in the suitcase.
Like many other factors when it comes to travel, it comes down to what works for YOU. LOL, as you can see, we all have our own approach to clothing, and all of it is "right". :O) When is your trip?