Who was the Roman emperor during the battle of Corinth?

No emperors under the Roman Republic;the Senate appointed 2 Consuls every year,and they generally led the army on campaign.However,as Rome began to expand from the end of the First Punic War (264 BC to 241 BC) onwards,more campaigns and thus more armies and more generals were needed,so non Consuls sometimes commanded.

Lucius Mummius was one of the Consuls for 146 BC,and commanded the Roman army at the battle of Corinth that year.

Julius Caesar never took the throne because he was never Emperor. He was dictator for life, dictator being an actual position within the Roman Republic at that time. Though usually it was a title held only for a six month period, long enough to get Rome through a crisis period after which the dictator stepped down and returned power to the Senate. It wasn't until after Caesar's death that his nephew and adopted son would become the first Roman Emperor as Caesar Augustus. Corinth took place in 146 BC so there was no Roman Emperor. The Republic was governed by the Senate. The last dictator prior to the battle was Gaius Servilius Geminus who held the position in 202 BC and the next one after the battle was Lucius Cornelius Sulla Felix who held it in late 82 through early 81 BC (there is a question as to whether he was appointed dictator in 82 BC or 81BC, but he stepped down in 81 BC).