I studied Mech Eng. and Physics but now spend most of my time building software related technologies. I think the big take away from my Mech Eng. degree is that I've learnt how to solve technical and non-technical problems. Solving problems is one of the most important parts of any startup and you should look at your Mech Eng. background not as a destination but as a continuous journey of learning to solve more and more problems (technical or not) as you encounter them in whatever challenges you confront. Apart from using your Mech Eng. skills directly in a mech related startup, those skills should not limit you to that specialization alone if you continue on the learning process of picking up more tools/knowledge to add to your chest of problem solving experience in a startup.