This was actually a controversy some twenty years+ ago on a science show in the Netherlands ('nationale wetenschaps kwis'). The wanted answer was that putting creamer in first was better as the resulting fluid had lower temperature and thus cooled slower. But a given answer was that the creamer made a top layer that made the coffee evaporate slower, hence keeping more of the warmth as well. During the show this answer was counted wrong. But (theoretically, I don't think an actual experiment was made) this is a factor working towards the same answer (first creamer keeps the resulting fluid warmer compared to adding the creamer later) that may actually have a larger contribution. So afterwards the consensus was that this may actually have been a better answer.