Short answer
Emotional maturity is an important practical consideration because marriage requires patience, communication and responsible decision-making.
Islamic Guidance
What Islam teaches
Islam encourages good character, patience, kindness and responsible conduct.
Practical Guidance
What you can consider
Look at how the person handles disagreement, criticism, stress, family issues and responsibility rather than relying only on qualifications or appearance.
Sources
Qur'an 3:134; Qur'an 41:34; Sahih al-Bukhari.