No for ASCII-art challenges, possibly yes for other challenges
If the goal of the challenge is to output a specific ASCII pattern, then the challenge's requirements in terms of output must be clear and precise in the OP.
If the goal of the challenge is to evaluate an answer to something, etc., then the way it is outputted feels less important, and too strict rules on the way the output looks might penalize some languages for no meaningful reasons. But the output should still be valid according to the OP if there are precise requirements indicated, and one should not have to think a lot to understand what the output means with regards to the challenge (e.g. parentheses or brackets around a list is fine, additional meaningless chars is not).
Some languages have additional outputs that you can't do much about
I'll speak for Prolog here but this probably applies for other languages: the Prolog interpreter really likes to output
false when you evaluate queries, even if you just want the value of a variable or if you want to print stuff to stdout with
write. There's not much you can do about it and as such those kinds of additional outputs should be tolerated.