An informed opinion is an unbiased opinion statement based on facts that expresses formal language and an academic tone.
An uninformed opinion is an opinion statement that may be biased, or based on personal experience that expresses informal language.
For example, an informed opinion on school uniforms:
School uniforms restrict students from expressing creativity in their outfits.
This opinion statement is based on facts, uses formal language, and expresses an academic tone.
An uninformed opinion on school uniforms:
I don't like school uniforms because they're ugly.
This statement is biased or based on personal experiences with school uniforms. The word ugly creates a non-academic tone, and expresses informal language.