I don't think 42 is terrible, but it is awfully average. This story is just okay. The sun monster gimmick really feels like it doesn't have enough energy to carry an entire episode, so things feel a little strained. Martha is pretty straightforward here in her story role and it's nothing special but Freeman Agyeman and David Tennant are both just kind of doing the best with the material at hand and it leads to a very watchable experience, to the credit of this episode.

In hindsight, there are a lot of traits to this episode that would ultimately represent the Chibnall era of Who, just like how Moffat's early writing credits were predictive of his time as show runner. We have an idea that probably could work with the right script, the characters feel like they should be more memorable as we spend a lot of time with them, but everything is kind of bogged down by empty dialogue, running from one plot point to the next, and little to no feelings of consequence or focus.

So it's fine, easy to get through, and not an unpleasant experience - I just sort of like Martha and the Doctor in any context, but I'm not going to pretend this isn't one of their more boring outings.