You're conflating two different aspects that need to be considered separately: attire and behavior.

In particular, I am focusing on your statement, "Stage actors and everyone else involved in the production of a live performance work really hard to put on their best show for the audience. The least we can do is show some respect by not looking like we’re headed to a street carnival."

Let's be realistic. The "stage actors and everyone else" on the stage cannot see you. There is no logical reason for your attire to affect their performance. You could be wearing a ball gown or yoga pants, for all they care.

On the other hand, they can hear you! If your phone is not off or muted and then rings, they can hear that. If you are noisily opening up candy, if you are talking to your neighbor, if you drop your bottle of water, they can hear you and it could be disruptive to their concentration and performance.

Speaking as a member of the audience who wants to enjoy a performance and not be made aware of others in the venue, it is critical to me that people shut the f* up during the show. If they can do just that, I don't care what they are wearing.