Is CMU a difficult school to get a good GPA at? Is it worth going to CMU over a state school (does the prestige help when it comes to medical school admissions)?
Generally speaking the prestige of your undergrad has approaching zero (but not absolutely zero) importance in medical school admissions. Your accomplishments during college–grades, MCAT score, ECs, LORs, leadership positions, oral & written communications skills, interviewing skills, etc–are much bigger factors than the name of your undergrad.
Attend the college that offers you the best combination of fit, affordability and opportunity.