Scene stealing NPC's

[Burley]

That's not necessarily always the case. I have one in one of my campaigns that I am dming because one of the players adopted a bandit from the very first encounter that at the time didn't even have a name. A few sessions later they positioned that NPC in a way that they became the focal point of the entire campaign because they came in contact with a certain item basically cursing him with immortality and the memories of the BBEGs.
At this point the npc in question is actively trying to escape this predicament that the party put him in with little success past getting stuck in a time loop paradox.
A NPC from the very first encounter of the game which only had the word 'cowardly' beside it as a description
is now a fully functioning member of the party which the only influence I had as the DM was allowing it to happen as events unfolded.

In another game a guinea pig with a headband of intelligence is a functioning member of the party. Also happens to be the only member with a positive intelligence modifier. If I recall correctly I believe the guinea pig was a leader of a nation at one point.

Would either of these for the bill for a DMPC? I personally don't waste much thought on it. They're in PCs that happened to share an abnormal amount of time with the party for whatever reason. As long as the player still have full control of the agency of their characters within the world then you don't really have to worry too much about them being overshadowed. If anything repetitive interaction with the same NPC and the changes that occur are a really powerful device to support this.

Okay, it's not always the case, but this isn't what the OP was talking about. Your examples aren't stealing the lime light.

In my current game, I'm playing an Artificer and the DM created a thief character who wanted to join the thieves guild, so, our party rogue helped her out with that and they started dating (DM and Rogue player are married). Suddenly, this thief character from the thieves guild is an Artificer, with a +10 to Initiative, has two subclasses somehow and always gets the killing blow.
I get that the class is fun and interesting, but it's what I, the player, am using. Let ME make it fun and interesting, please. (My artificer Niall is a gnome, who has no magical ability, so, everything is alchemical. Except for his pet pig Hammers, who his brother accidently turned into a cauldron.) I can make interesting characters, but I can't compete for screen time with the person who literally runs the world. Now, even though the DM doesn't have any plot hooks or adventures ready, this DMPC want's to join our group and go adventuring, leaving behind the thieves guild we busted to get her into and the orphans she joined the guild to support.

That's the DM being new and bored and wanting to be a player again. That's where the line between NPC and DMPC is crossed.