To give two sides of answers to your question: I played the early MK games on the Sega Genesis, and I would say start where you want.
As others have said, the new canon for the series starts with the 2011 game. But honestly, the 2011 game is very similar to the OG games. If you only played the OG, then the new games will only be a better version of what you played up until that point.
If you only played the new ones, it gives prominent backstory to various characters, and the gameplay is almost identical. It’s a great starting point for newcomers.
Biggest difference is that the new games try to make everything one seamless story, the idea being comic book multiverse things, where in one universe, Ryu isn’t the God of Lightning, but a simple student at the temple. Stuff like that.
TL;DR: There’s no bad place to start in the Mortal Kombat games.
To give two sides of answers to your question: I played the early MK games on the Sega Genesis, and I would say start where you want.
As others have said, the new canon for the series starts with the 2011 game. But honestly, the 2011 game is very similar to the OG games. If you only played the OG, then the new games will only be a better version of what you played up until that point.
If you only played the new ones, it gives prominent backstory to various characters, and the gameplay is almost identical. It’s a great starting point for newcomers.
Biggest difference is that the new games try to make everything one seamless story, the idea being comic book multiverse things, where in one universe, Ryu isn’t the God of Lightning, but a simple student at the temple. Stuff like that.
TL;DR: There’s no bad place to start in the Mortal Kombat games.