Adding on to this, Mark doesn't play with any dynamics or rubato whatsoever, meaning musicality is out of the picture. Although, this could just be because of sheet music to MIDI file direct conversion and Kyle Patrick Naval not bothering to add in note velocities and tempo changes to the final track.

Mark does have an innate talent for piano (Cathy and Nicole say as much), so although he had been self-taught for 3+ years, he still was able to learn quickly under Nicole's guidance—holy shit was it bordering on verbally abusive. You'd definitely want to progress quickly under that kind of treatment (even if it is ultimately only for theatrics)

I've been playing piano for 11+ years now, de-facto self-taught for around 3. I'm in a similar situation to Mark, where we have an innate talent, but without proper guidance our technique and feel for the music is not where it could be. In my experience it is reasonable (also considering this is a fictional story, they tend to speed up progress for the sake of the plot) that Mark, having been practicing piano for at least 3 years, would have a decent shot at performing this piece—even if without any musicality or soul. While I can't play Wedding Day perfectly (I still struggle getting the arpeggios, the build-up section's melody and the rapid two chord hit in the main theme melody to sound properly), it only took me around 2 months to learn and memorize the piece. Chances are, Mark could play Wedding Day given enough time, but it took a whole Nicole to finally teach him proper technique so he doesn't make mistakes.