Shortcode Generator plugin creates unwanted tags in content

I don’t know about the plugin, but it seems that wp is filtering that content with the wpautop filter (which, among other things, puts p tags at the beginning and end).
Search the codex or forums for wpautop, many people have asked already how to get rid of the p tags.

Just use a free online one like at https://pythoughts.com/wordpress-shortcut-code-generator-useful-shortcodes