Is a day trip from Kotor to Dubrovnik feasible?

The few times I have crossed the border between Montenegro and Croatia it has taken about 30 minutes. I am a little confused by the second half of your question as if you are staying in Kotor, yes, you should explore it. Beyond that I guess a good answer would need to be in response to all of your plans and I suspect there is more than you going to Kotor or Perast.

We will be flying into Podgorica in the evening and staying in the Kotor/Perast area for three nights, with a rental car. Our plan is to explore Perast, Kotor, and also visit Sveti Stefan, so Montenegro is definitely the primary focus of this part of the trip.

That said, I have come across quite a few posts suggesting Dubrovnik is especially impressive (some even comparing it favorably), which made me wonder whether it is worth including as a day trip, if only to avoid feeling like we missed something nearby. I am trying to understand whether Dubrovnik offers a distinctly different experience compared to Kotor/Perast, or if the scenery, old-town atmosphere, and photography opportunities are similar enough that focusing solely on Montenegro makes more sense.

Dubrovnik is a wonderful city. Almost unique. BUT, you have the opportunity to Montenegro this trip, use it, Three nights isnt much time and you can cover a lot more ground if you travel south down the coast and see Budva, Stari Bar, Ulcinj or go inland to the monastery at Ostrog. This time that, I think, would be a better trip.

3 nights means you hsve 2 days in Montenegro. I wouldn't mess with a car. Stay in Kotor, spend one day with some time in Kotor and a boat tour. Another day hire a guide and go down the coast. The P1 out of Kotor then Cetinje, Budva and Stari Bar at least.

Alright, I will your suggestions. Thank you