IIRC, plastic surgery can cost around 300,000-1.5 million yen (roughly $2,000-10,000 USD), so it's possible Mikoto doesn't want to burden her mom with the surgery costs.
Okay but genuinely curious, why is it that the MC's friends/family members (both the male and female's) do not just tell them that their feelings are mutual? Even though it's literally in front of them?