it's not a plot device. look. let me spell this out for everyone so it'll be easy to understand.
1. miku works hard. specifically, she is very earnest about self-improvement.
2. fuutarou knows this about her. he's spent enough time with the quints to get subtle aspects of their personality. fact is, ichika might actually work harder than miku given her dream to be an actress, but it's just not quite the same kind of 'trying.'
3. miku's love and effort in baking got through to fuutarou in spite of the relative mediocrity of the bread. aka, it has nothing to do with the bread you fags -- otherwise fuutarou would've said, wow nino you know you're good at baking like my mom was.
4. hence, the bread was just a vehicle for fuutarou to realize huh, fuutarou kind of reminds me of my mom's motherly affection.
it's oedipal shit you dumb fucks. but if you say it out loud it gets weird