the lisp printing is on the same line

Indeed, print prints a newline. princ does not.

To further control how you print stuff, there is format. It works with directives that start with ~. Thus, ~a (or ~A) prints the object given as argument æsthetically. ~& prints a newline.

(format t "~a~&" 'a)

t is for *standard-output*.

See https://lispcookbook.github.io/cl-cookbook/strings.html#structure-of-format

This link also explains how to align strings to the right or to the left, which you might need (if you go the format route).

You can make your code shorter. There is make-string count :initial-element character that can replace the loop. A character is written #\a.

Also you could call triangle recursively only if k is > 0 (zerop). With this and (format t "~a~&" (make-string k :initial-element #\a)), my version has only one call to the print function and less “if” machinery:

(defun my-triangle (k)
   (format t "~a~&" (make-string k :initial-element #\a))
   (unless (zerop k)
     (my-triangle (decf k))))