Should I tell my friend I don't like their new partner?

It's not always a straightforward yes-or-no situation. If you're close friends, they might eventually notice the tension anyway, so staying silent could breed resentment. But there's a big difference between sharing genuine concerns if you see actual red flags versus just not clicking with someone - in the latter case, keeping it to yourself and being cordial is probably the move. If you do decide to say something, frame it as care for them rather than criticism of their partner, and only if they ask or if you're genuinely worried about their wellbeing.