This whole argument is kinda weak, to me.
Far from a Bruno Mars fan, but Mark Ronson said this track was an homage to songs that he grew up on and loved back in the day. He's a DJ. He said that he wanted to salute his favorite period in music by putting this track together. It's not like dude is acting like he played all these instruments himself. He straight up said he took these parts and wanted to make something that reminded him of back in the day.
He's sampling. This is pop music, so I get the hate, but Hip Hop was birthed on doing the exact same thing. Peep older EPMD credits. Erick would take 4-5 different records that were mad popular in the 70's and make a track off samples that everyone knew. This is really no different. I know it's a Bruno Mars joint, but it's all the same shyt that's been done in music forever. The rules don't change because this is some pop shyt. LOL!!