It's not about the line itself, but more the sentiment behind it. The fact that the Emperor is just suddenly back without any buildup or hinting in the previous two movies is the problem.

It makes sense that the rebels didn't know about it. It doesn't make sense that the first the audience hears about it is that line. It feels lazy. They could have mentioned, in an offhand way, that the remnants of the Empire is pursuing cloning tech. Not only would this tie the final trilogy to the second trilogy. (First? Episodes 1-3, anyway) But it would also make that line make way more sense.

As as much as the Thrawn trilogy feels like bad fanfic, it does tie the whole clone wars/rebellion thing together, and features someone who comes back as a clone. I think it would have made a way better trilogy than what we got.

Because it was just saying out loud what hollywood writers have been doing on movies for a while. "somehow this movie happens. Just pay us."

The problem is the movie didn't show him returning. Instead they're just like hey Palps is back! With absolutely no lead up or anything. They should have actually shown the message or whatever it was he sent out to get everyone all worked up in the first place. Then that line wouldn't have sounded so cheap.

I can only speak for myself, but it wasn't so much the line as the hand-waving that came with it. It was more that I found the line relatable, but you're right about it being appropriate for the scene.