It's hard to deny that Star Trek: Strange New Worlds' musical episode, "Subspace Rhapsody" was a good idea. The episode is one of the worst-reviewed episodes of not only the series history but of the franchise's. It's seen as a down point for the otherwise pretty successful Strange New Worlds series and only one other episode is more poorly reviewed for the show; "The Elysian Kingdom".
Musical episodes are usually a dying gasp from a series, a cheap ploy to get viewers back into the show. The only problem is, unless the show already is heavily incorporated with music (see Glee), all you end up doing is pushing the fandom away. Save for a small sect of fans, most Star Trek fans never wanted a musical episode. Whether you like it is entirely your own personal preference, but the score the episode has on IDMB is indicative of how many fans didn't want it.
If only the series had a Tom Welling to speak up. While filming the hit CW series, Smallville, Welling was presented with doing a musical episode for their series. He absolutely refused to be a part of it. Speaking on the Talk Ville podcast he shares with former co-star, Michael Rosenbaum, Welling revealed that Smallville higher-ups really wanted to do a musical episode. Something he refused to be a part of.
The episode in question was the season six offering, "Noir", a story that focused on Jimmy Olsen stuck in a dreamscape of sorts that resembled the 1940s. A concept that's already a death knell for an episode. Yet, Smallville execs wanted to further turn off fans by making a bad idea worse by infusing it with a musical concept as well. Welling outright refused to show up to film it, saying;
"I literally was like, 'I will not show up to work. You can film it, but...'"
The show would get four more seasons, ultimately ending with season 10 in 2011. The "Noir" episode is considered one of the franchise's worst and adding a musical element would've only made it worse. Star Trek isn't in such a good place that it can afford to have the one series that is reaching non-Star Trek fans regularly turning off fans but using such a wildly unpopular gimmick as musical episodes.
Look at The Flash, whose musical episode is among its worst-rated episodes of all time. You can maybe get away with this in a comedy, but for the most part (barring a few exceptions), musical episodes are often rejected by the core fandom at a resounding rate.
If only someone on Strange New Worlds had stood up and said "Musicals aren't Star Trek, I'm not doing this" then maybe we would've been saved from such a poorly received offering.