You’re not gonna be able to display an ASCII table if the font isn’t monospace. That means you can’t display it with the regular Slack font, but you can use Markdown formatting in Slack, as you can here on the forum.
Markdown lets you make use of backticks to turn a text into a code block - which has a monospace font
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like this
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If you don’t care about being able to copy the table contents, maybe you can build a JSON (dictionary) instead, and pass it to an API that converts it to an image, then display the image (or send the link to it).