The first 2 games were not bad for their times. Fighting games were still pretty simple and MK offered a similar experience. The only things they got wrong was standardizing the normals to the point that only a few characters had any unique ones. (Though the single player game had a lot of balance issues and cheating, albeit less than later on. That also existed in other fighting games of the time, but MK was always the king of unfair single player mode.)
MK3 is where it fell apart. Gameplay got too complex; you had to memorize too many moves with unique inputs to the point that gameplay was confusing. And certain moves would often just not work; Sheeva looked like she would be OP, but she whiffed 90% of the moves that Goro & Kintaro would never have at that rate. Anything teleportation based either unbalanced the fight, threw you into the opponent's attack, or both. And then there's the roster: Tons of fan favorites thrown out for dull newcomers, and even returning characters felt wrong. (Sub-Zero being turned into discount Henry Rollins and Shang Tsung being a white guy were some weird ones.) And single player got worse: the CPU abused the Run button and combo system (always perfectly done with the best combo for them, but not you) to kill you in seconds.
And you'd think UMK3 would fix everything, but it didn't. Yes, the roster added back in some fan favorites. But old characters weren't rebalanced, and the single player difficulty was actually made even more unfair. Which brings us to the worst thing in the game: Jade. She was the culmination of everything wrong with MK gameplay. Cheater CPU, cheat move to invalidate your moves, and unfair priority on her moves. One match against her, and you were convinced the whole game sucked. I had taken the time and abuse to play through MK3; I refused to complete UMK3.
Other than that, MKT was an interesting novelty, but not worth more than a rental. The games never matched the quality of MK2 again.