First, there was a song called "君のひとみは10000ボルト" --- your eyes are 10k volts. Wikipedia says it was often mistaken as 100万ボルト,probably because it's more catchy and as smooth to pronounce as the original. I'm not sure how trustworthy wikipedia is in this regard, but I wasn't sure it was 1万 or 100万 until I looked up to write this answer.
The idea is, I guess, imagine your eyes meet those of someone you care for (or imagine love at a first sight). Everything in you reacts, instantly. Your heart pounds, your mind filled with emotion. That's like a lightning or electrical shock with huge voltage. So strong, be it 一万, or 百万 volts. The number is arbitrary, but if I were to choose, 100V is what electrical taps provide, 1000V sounds weak, 一億 V sounds rather silly.
I'm younger than the generation but still know the song, and this reference to voltage in the eye, especially 1万 or 100万 is alive elsewhere. Your "幼女の瞳は百万ボルト" would be an example. 百万ボルトひとみ is a ひみつ道具 (tool? invention?) from Draemon, which apparently is also an example. The story in the anime version is described in the official website.
For the comic strip, read it like this. (The translation is rough and cumbersome but I guess it gives you the idea)
- Nobi is staring into the mirror.
- Doraemon: "Your face is not that good, enough for you to look at fascinated." "What are you doing?" (= "What are you doing? You aren't surely admiring your face because you don't look that good").
- Nobi: (Reacting to Dora's critisism to his looking) "Why is it bad having a bad (=ugly) face!?" (= "What's wrong if I looked ugly!")
- Dora: "Well, if you have these, any man however hopeless will be popular among girls". (もてる here has a special meaning)
"Even you".
I don't think 幼女の瞳は百万ボルト and this comic strip are connected: they just stems from the same song.