蛙 has ア【a】 as one of the on-yomi, and kanji compound is usually read in on-yomi (some also have a reading in kun-yomi, but in general, all of the kanji in the kanji compound are read in either kun-yomi only or on-yomi only, but not mixed).
Some example where 蛙 is read as "a":
蛙黽 (abou): tree frog (toad)
蛙鳴 (amei): frog calling
蛙鳴蝉噪 (ameisensou): ...
I find that this source gives a relatively accurate description of the way Midorima uses the phrase: "Nanodayo" (...) adds a tone of 'an explanation about something that doesn't really need explaining' to his words."
'Nanoda' is a form of the verb "to be"（in this case simply 'da' / だ）, used to indicate assertion / a 'decisive' tone, whereas yo / よ gives a ...
The Japanese version of this meme could be considered:
My power level is 530,000
Which is close enough but didn't originate from the same scene. The joke also propagates to any score measurement in any setting. For example one can brag about their TOEIC (Test Of English for International Communications) score:
My TOEIC score is ...