Can people explain Doomsday to a casual viewer?

You have perhaps noticed that Jonathan Majors was involved in some legal troubles. I hope this answers at least some of your questions.

Due to Jonathan Majors the actor who played Kang. They pivoted to Doctor Doom. We do not know if Kang is gonna be in Doomsday or not. That story wasdropped.

nowhere. the actor who was playing kang, jonathan majors, got some heat for alleged domestic violence and was dropped, so they totally rewrote doom as the main protagonist instead. we don't yet know what his movie origin will be, or why he's being played by RDJ but there are loads of theories.

I'd just watch the Loki show. That's all you need in that regard

Majors got into legal trouble which led to Marvel severing ties with him and apparently (just assuming) led to a change in plans.

A few variants of Kang were involved in the Loki series, which I recommend highly. I don’t want to spoil here, but if you want to just have a quick read check Wikipedia, but a watch is worth your time.

Think about anyone you haven’t seen in awhile. It can be anyone. Just imagine that person in your head. Think hard. Got it? Okay.

That person… will return in Avengers Doomsday

You'd be in a very specific minority of casual viewers who remember the Kang storyline but aren't across the pivot to Doctor Doom. Doomsday will probably provide enough of a handwavey explanation to address it.

Kang wasn’t working and the actor got in trouble so they changed it to Doom. Based on how they described him, he is played by RDJ but is still his own character that is separate from Iron Man.

Kang was originally supposed to be the next main big-bad of the MCU even having the first Avengers movie after Endgame being dedicated to him called "The Kang Dynasty," but controversies surrounding Jonathan Majors screwed it all up and his story was completely scrapped along with "The Kang Dynasty" in favor of "Doomsday" with yours truly.

Kang was supposed to he the next Saga villain.

He was set up majorly in Loki S1 and Ant-Man 3

Jonathan Majors did a big oopsie and went to court and subsequently got fired

Loki S2 went “don’t worry we handled it”

Now Doom is bad guy

Why not just wait until the movie comes out and you’ve seen it before asking questions?

Yes it is a movie that will come out later this year.

Doomsday is a monstrous DC Comics supervillain, infamous for being the creature that killed Superman in the 1992 "The Death of Superman" storyline. He is essentially an unstoppable force of destruction, driven by a primal hatred for all life.

Basically a combo of Ant-Man 3 being poorly received/underperforming and the actor who plays Kang getting into some legal issues and being found guilty, led to Marvel scrapping the entire Kang story and pivot to Doom at the last minute

Since Avengers Endgame Sam has taken the mantle of Captain America and is (finally) putting together his own team. This team probably involves Ant Man, the new Falcon and Shang-Chi.

But, he's not the only one using the Avengers name. A lady called Valentina (basically a corrupt Nick Fury) accidentally led to her mercenaries forming into a group called the Thunderbolts to take down a superman-like hero called The Sentry. She spends the situation, making them (including Bucky and Yelena) into the 'New Avengers', residing in the old Avengers Tower.

The New Avengers in their post credit scene spot the ship of the Fantastic Four, which has somehow entered the 616 universe. We see from the Fantastic Four movie that Doom arrives and seems interested in their son Franklin, so perhaps those two scenes are connected.

In general, the multiverse has been a growing existential threat for our heroes. Doctor Strange faced incursions (when two universes hit one another) and is currently MIA. Captain Marvel lost Monica Rambeau in another universe. The leader warned Sam of threats from beyond our universe. All this stems from Loki, who overthrew a variant of Kang at the TVA, with Loki now in charge of the endless branching timelines.

That's all we have leading into Doomsday. We also have Thor, who is out in the universe raising his adopted daughter, Love. We also have Shuri as the new Black Panther, meeting up with one-time enemy Namor.

That's what we know. Obviously a lot of setup went into Kang, so time will tell to see if that's addressed or not. There was a line in Loki Season 2 that talked about clearing up threats from the timelines, so perhaps Kang is dealt with and done.

It's easy to explain.

Doctor Doom is the bad guy.

The Avengers, X-Men, Fantastic Four, Thunderbolts, and others are the good guys.

They will need to presumably work together to defeat Doctor Doom.

Kang dropped without resolution. Doom in as villain.

The Kang dynasty was resolved in Loki Season 2. Doom is the big bad. Not always the plan but it’s where we are.

go and watch it