There are many people who claim that Luffy is not Yonko material yet but if we look at it unbiasedly then we will come to the conclusion that he deserves that title. Here i will try to persuade you.
1- He deserves it since he might have lost a battle but he won the war. He went to Big Mom’s territory, completed his objective and returned back safely after turning her kingdom upside down.
2-Those who say Luffy didn’t defeat Big Mom should consider what a Yonko stands for. In my opinion Yonko is someone whose name is enough to instill fear in other pirates but if a pirate goes to a Yonko’s territory and comes back alive then he should be considered powerful enough to challenge a Yonko and a Yonko’s challenger can only be another Yonko.
3- Also if you look at the fact that Luffy is the captain of the crew who turned the main base of a Yonko whom even the military is afraid of upside down with just the half of his crew then considering the fact that what would have happened if the main fighters of his crew (like Zoro, Sanji-because he was mostly captured-, Franky) were there.
4- You can also consider that only those members of the crew which had a bounty of less then 100,000,000 Berries were at Whole Cake Island except Luffy and Sanji and still they did pretty well.
5– When you consider Luffy’s bounty then you will see that we haven’t seen any character whose is not a part of a Yonko’s crew having a bounty even near to that of Luffy. The Marine probably also considered the fact that he is forming alliances with the Pirates of the Worst Generation and as he is kind of leading the Worst Generation so if the majority of them gather under his flag rather then joining the Yonko’s then it will cause serious problems and undermine the delicate power balance between Pirates and the World Government.
6- I will also mention that I don’t think that Luffy is as powerful as a Yonko but he does qualify as low level Yonko that is the strongest of all the other pirate crews which are below the current Yonko’s in strength and power. Also he has a large fleet of pirates following him, so that also adds to him potential of being an Emperor of the Sea.
*Theory by 99cent Hitman