Assuming you are taking the Luzern route to Interlaken, not the Brig-Spiez route:
The EC trains between Milano Centrale and Luzern do stop in Lugano. These trains use the deep Gotthard Basis tunnel between Lugano and Luzern. If you want to avoid the tunnel, you can take the Gotthard Panorama Express from LugNo to Flüelen and chNge there to a regional train to Luzern. This train has only one departure a day and I believe it is all First Class.
An alternative, if you want to break up the journey to Interlaken, would be to go from Milano Centrale to Locarno on Lago Maggiore, and overnight there. Then take the TrenomGottardo regional train to Luzern the next day. This one uses the same tracks as the Gotthard Panorama Express and avoids the deep tunnel.
Whether either city is “worth”an overnight stop is a question I cannot answer, as we have not been to either Luzern or Locarno. But I do like the look of Locarno and the availability of lakefront hotels just blocks from the train station (unlike Lugano). I have planned several itineraries that included Locarno in our trip between Italy and Switzerland, including one this September, ut for various reasons I had to make changes and drop Locarno. I still hope to get there someday!