5'7" @ 245 too short for MMA heavyweigh?

For your height, you need to lose some weight first. It's more than explosive strength, you need some endurance. Even if you say that the fight won't last that long, every second feels like a long time when you're wrestling somebody, especially when you're supporting the opponent's weight also at that moment.

I'm not saying get down to your BMI scaled ideal weight, just lose the amount you'd lose from increasing your endurance. Controlling your intake on food is important along with training of course. You'll get your reward when you see your run time improve, feel better after a workout, feel less winded during sparring, higher metabolism, and lower blood pressure.

I forgot to directly answer your question. No, you're not too short for MMA Heavyweigh.

There is no rule against being 5'7 and fighting heavyweight in mma. However, that would be overweight for most people that size. most of the heavy weights will be much taller. If you have the skills you can overcome the height differences.