Like all other Sims 4 Expansions, Get Famous has its own cheats. There are a few pitfalls to avoid however, so let's go over how you raise fame, public image for your celebrity and give them promotions in the acting career.
Enable Testing Cheats
Hit Control + Shift + C on your keyboard, then type testingcheats on - this is necessary to do many of the cheats listed on this page.
Increase Fame (or Decrease Fame!)
Shift-click a Sim with testingcheats on and select Public Image - from there you can modify their fame. Freeze fame, increase fame, and decrease fame are all available.
Change Public Image
Similar to above, this same menu will allow you to modify a Sim's public image. There are 7 levels. 4 is neutral, 5-7 are positive and 1-3 are negative with 1 being the lowest it can go.
As celebrities gain fame, they are going to gain quirks. It's an unavoidable thing. The best you can do is to try to get quirks that you can live with. I found my Vain Street quirk to be extremely annoying, because the Sim would get tense multiple times per day. The Public Image menu mentioned above can also control this, though it's worth mentioning that you can remove fame quirks without cheating by purchasing a reward in the store. Once the slot is empty, the same quirk can come up again, so make sure you direct your behavior elsewhere so you don't get the same one.
Cheating to raise the acting skill is a very simple matter of using testingcheats then stats.set_skill_level major_acting 5 for level 5, or stats.set_skill_level major_acting 10 to master it. On the other hand, cheating your Actor career levels, that can cause problems.
If you cheat your career level with an active audition or role ready, you can get completely locked out of getting new gigs. When you cheat the game won't show the gig any more, so cancel it using the phone to get a new one as I'm not sure if the gig will still play out normally. Once you are in the clear, careers.promote actor is the simple cheat to raise your acting skill. If you type it, then leave the text box open you can push the up arrow on your keyboard, then enter, to repeatedly raise this skill (some people don't know about that).