Your statement is complete syntactic nonsense and probably failed with an error message like:
-bash: syntax error near unexpected token `do'
The shell recognizes the first word of the command line ("dialog") as the command and everything else up to the first statement separator (";") as arguments to that command. In order to insert the output of one command into the command line of another, you can use command substitution, or "xargs", provided that you can add the output to the end of the command line.
dialog --checklist $(commands)
commands | xargs dialog --checklist