I've been reading a manga and there was a sentence:
Can someone tell me what does it mean?
As the question still is open, I'll try to answer it.
it's waste of illustration so don't show yourself.
is very much off.
This is the 'pensive' or 'monologue' な, not the imperative な
コマ means 'frame (of a comic)'
This is the particle から ('from'), not 〜だから ('because')
Do check the dictionary for 出る (and/or 出てくる). Here it means 'to come out'
'Waste of xxx' is typically 「xxxの無駄」
無駄に, when used this way, means 'pointlessly' or 'for no purpose'
A correct answer would be (if we consider it to be the pensive な),
[something] is coming out of the comic frame for no good reason
'Something' here, I presume, is referring to whatever is in the frame that is sticking out.
If we take it as a command, it's
Don't stick out of the comic for no reason!
"Don't stick out of the frame pointlessly" or "Don't stick out of the frame so much."