Actually... from what I could gather, he seems like some kind of common soldier. He has the looks of one, if this image is anything to go by:
You can see on both sides that there are guards with silver helmets and their clothes are exactly the same as the ones human/young Mummy have in the past. These soldiers/guardians are the ones making sure the common folk doesn't step out of line.
Then we got Paititi lore (finally!) of all things that those three in the front ARE the royal family of the city. The child even intimidated an alive Mummy as seen in the little video I posted above.
The most interesting thing is, those guards who are apparently the ones that keep the safety of the royals and are (literally and metaphorically) way above the citizens and the other silver-wearing soldiers, are using the exact uniform Mummy has in the present.
(As to who are the two men in the back, I suppose they are the high priests or something like that, but very important people for sure)
Here's a closer look on both uniforms side by side. Also, is it just me or Mummy seems to be a sort of pariah in his community? He gets pushed around and has no respect even though he had authority in the undead City of Paititi.
Perhaps, just perhaps, him remembering that everyone loved him was him idealizing the past OR maybe the people love/respect the human and bully the mummy (which in Incan culture, that would be a BIG NO NO as they are sacred beings, but oh well, the royal family look like Mayans instead of Incans, what was I expecting?)
As to why he now has the uniform of the high/royal guard, perhaps he climbed the ranks between the past (1502 as the official sources say the group traveled) and when the Spanish Conquistadors arrived to the Inca Empire (1532). I mean, 30 years of difference would be enough for the young man to prove his worth and earn that high rank, as that's what happened in the Tahuantinsuyo and other cultures as well.
What would be the butterfly effect that will change everything we know? Well, apart from traveling and altering other time periods, it seems that Tad and Mummy will do something SO stupid (maybe they admited out loud the young prince's music is awful) that the royal kid will become ofended and his daddy intervened, as he says in the Spanish trailer "No one dares to insult the crowned prince!" And that's why these two are in that situation above and young Mummy would lose his opportunity to become what he is in the present, of climbing the ranks.
As to why the royal family would be mad at human Mummy? Simple, he was the one to bring the group to Paititi as prisioners (althought Tad arrived with Oli and Jeff on his own) so he's responsible for the outsiders doing chaos in the ceremony and he's the one to fix the situation or else, and that's why he apparently goes with the group to time travel, because he was chasing them at first, then mellowed out when he realized the royal family couldn't hurt/punish him anymore.
In summary, as far as it has been shown, Mummy started as one of the common folk, then gained the low-ranking soldier job, then obtained the high-ranking guard job decades later because let's admit, meek attitude regarding royals aside, the young Incan is pretty badass.
And in the present, Mummy is still in that wild phase of doing whatever the heck he wants after being stripped from his job and wanting to relive the glory days where he was truly happy... except that he isn't. Young Mummy lived in fear of the royals, high-ranking soldiers probably bullied him as they did to his undead self even if he had the same rank as them by that point.
Perhaps that's where all his issues come from, the feelings of being unwanted, of taking offence when shouting at him that he's a burden, of demanding respect in the third movie when Ann told him he was gross. He had enough of being pushed around, being seen as a lesser and following orders.
Also that explains why he never prided himself (at least in canon, I'm not counting the educative shorts) in being someone important in his society to Ramona or his friends, because he wasn't THAT important to begin with.
Or worse, even after gaining the high rank and having his own troops to command, years of living in fear as a low-ranking soldier, possible intimidation, losing his sanity underground and seeing how everything he archieved both in life and death crumbled when he let Tad and co. go, made him think he isn't important at all.
All the validation he has right now is of those who were spared of death by him, Tad and Sara, and now they have all their attention and love on Olimpia, as she's their daughter.
And what does Mummy have without that attention? Apparently nothing. Only the pets (who can't talk back to him, and that's why he made the wish for them to start talking so he wouldn't be alone in conversations), Ramona (who is, well... Ramona) and a few Instagram followers he can't really interact with in person or they could discover what he really is.
The worst irony of all is that, there's no other living being in the whole world that loves Mummy inconditionally as chaotic/messy as he is than Olimpia, the little girl Mummy is so jelaous. I hope that he realizes than he doesn't need to go to the past to be important again, because he's already someone worthy of being loved, royal or not.