For most MD schools, no — it's a meaningful plus but not a requirement. For research-heavy programs, it's close to an expectation. For MD-PhD programs specifically, research is the core of the application, not a supplement to it.
If you do pursue research, stay with one lab long enough to genuinely understand your specific project and be able to discuss it confidently under questioning. "I pipetted for a semester" doesn't impress anyone on an admissions committee. Being able to clearly explain your hypothesis, your methods, and what you'd do differently does.