That’s the “problem” with manga. It’s not text characters that can be selected, highlighted, and searched. It’s images, which (for a computer) is no different than trying to select text from a photograph of a flower.
If you have a desktop computer or laptop and the Chrome browser, you can install Copyfish and use that to try and read the text from the Kindle cloud reader web site. The extension’s been hit-and-miss for me, but it seems to get it right fairly often. It’s what I’m using on 「ひとりぼっちの○○生活」 when I hit a kanji I don’t know, and it seems to be able to recognize kanji for me most of the time.
Here’s an example of its use on a manga page I brought up on a web search:
I think it can work with the Kindle app on desktop/laptop as well, but it needs a companion screenshot app to be installed as well.
If you haven’t tried this method, but are interested, check the “Manga” section of this guide for installation and usage steps: