Convert text to "spongebob' format from your command line ...

spongebob

#!/usr/bin/env bash

set -eo pipefail

[[ -n "${VERBOSE}" ]] && set -x

read text

while [[ $index -lt "${#text}" ]]; do
  swap=$(( $RANDOM % 10 ))
  char="${text:$index:1}"

  if [[ "${swap}" -lt 5 ]] ; then
    echo -n "${char}" | tr '[[:lower:]]' '[[:upper:]]'
  elif [[ "${swap}" -lt 3 ]]; then
    echo -n "${char}" | tr '[[:upper:]]' '[[:lower:]]'
  else
    echo -n "${char}"
  fi

  index=$(( $index + 1 ))
done
echo

Usage:

1. Save to your local file system: ${HOME}/bin/spongebob.
2. Grant the file permission to be used as an executable: chmod a+x "${HOME}/bin/spongebob"
3. Ensure it's visible on your system's $PATH.
4. Now run echo 'This is SpongeBob text!' | spongebob from anywhere.