It is also a much better and scene than the one I thought I was getting, but then most Perrin developments ended up like that for me. (I thought he was going to be used to represent of all the oppressive Chandrilan traditions that are smothering Mon, but she's the one in their relationship with the higher status and influential job and family wealth, not him; there's that scene in s1 where Mon and Vel are discussing Leida doing all the Chandrilan traditionalist stuff and Vel immediately assumes it's Perrin's influence but Mon says no, he's "surprisingly open-minded", it's all Leida.)

I also thought initially he was pro-Imperial, but he's not, really; he doesn't seem to have any political affiliations or leanings at all any more although she says he was once "the academy firebrand" and he isn't happy she's brought it up:

Right after this they go back to talking about what Mon's doing at the party, which is trying to gather votes against the new sentencing legislation. Tay says that the legislation is having an effect (which we know because the storyline's cut with Cassian getting sent to the prison on Narkina due to that same legislation). All Perrin says about it is "the rebels should have thought that over before blowing up Aldhani" before leaving them to it, and so he doesn't sound like he particularly cares, but he's also hit on the same point Mon brings up with Luthen about Aldhani not long before - "Do you realise what you've set in motion? People will suffer!" - and not long after this, one of the Senators whose votes Mon is after turns up to talk to her having been steered her way by Perrin.

I don't think Perrin is secretly pro-Rebellion here, or that he cares about the sentencing directive more than he's letting on, but I do think that what initially looks like it's going to be Tay the principled rebel vs. Perrin the disinterested hedonist isn't even that simple at the start.

Back to the car scene: to me it makes Perrin more interesting than he'd be in a scene where he's being confronted over something he has done, because this way you get to see his reaction - anger at Mon for not believing him wrapped up with warning her that someone's lying to her and using him to do it (and he's correct, he just doesn't suspect her).

But it makes Mon more interesting, too! There wouldn't be much of a moral compromise needed in blaming Perrin for something he'd genuinely done; the decision she's making in blaming him for this is a tougher one, and it's obvious she feels that. And that the subject of Perrin and gambling is so personal as well, that this is something they both acknowledge he's done before in a way that caused her significant... pain? difficulty? she uses the word 'shamed' which makes it a lot more emotionally loaded than just losing money; that he promised he wouldn't do it again, and full-on shouts this time that he's "kept my promise!"; that all this matters to Mon; and that this is still a sacrifice she's prepared to make for the greater cause.