Dr. Mario is a classic recurring character in the SMG4 series.
He is a fake doctor who mainly comes when there is a disease. Even though Dr. Mario actually isMario (and he even addresses this in the episode Yoshrooms), he is sometimes treated as a separate character. Whenever it comes to his job, his actions in "treating" patients are futile, since he had a fake degree and hardly cares for others.
There are 2 stories about how Mario became Dr. Mario:
The first one is in the episode R64: A Dose of Dr. Mario, where he did an exam but failed. Despite that, he became a doctor.
The second one is in the episode SMG4: Mario University; While he was teaching to some kids, a medic had a stroke and Dr. Mario (Mario at that time) replaced Dr. Wily as the doctor and gave the collapsed medic hundreds of pills. Obviously, that didn't work and the medic died. Mario then writes himself a fake degree.
According to the video R64: A Dose of Dr. Mario, he is a very bad doctor, not even a real one at that as he failed his medical exam. In the episode "Yoshrooms", in which he and Mario are separate people, he stated to Mario that he was just his doctor version.
He also does not care about who he heals, even if they are pure evil; this is proven in the episode "The Babysitters", where he heals Pepsi in his monster form after he is speared by Mario. He does not care very much about his patients in general.
Mostly when he appears in recent videos, Doctor Mario WITH Lyrics by Brentalfloss is heard; the song is so far Brentalfloss' only work to appear in any of SMG4's video's.
It is also possible that this song is the inspiration for his behavior.
He is the best doctor in the hospital he works at, as shown in R64: An Overdose of Dr. Mario. This can either mean that the other doctors that work there are worse than him or it is just because it is an SMG4 video (and therefore, a joke).
From SMG4: Luigi's Lesson onwards, Dr. Mario now has a fan-made or a new custom SM64 model. What separates this from the original is that Dr. Mario now has his coat, stethoscope, and other requirements.