In the Yotsuba-to manga one of the heroes first calls a younger of the two sisters (16 y.o) 美女. Then he gets to know an older sister (I guess 21 y.o) and tells that she is a real 美女. A younger sister hearing that gets somehwat distressed and he then tells her that she (the younger one) is a 美少女. This, as far as I can tell from the picture creates a comic effect. However I was not able to find a comic connotation looking up the 美少女 word in a dictionary. So, does the word 美少女 have any ironic or humoristic connotations?
少女 is simply "girl", making it clear that it's a young girl (compare 少年 for boy).
In this example, by making the adjustment that the younger sibling is actually a 美少女 rather than 美女, it points out that she's the younger sibling of two, and not being a FULL beauty to the extent her sister is. It also hints at her having a more young form of beauty than her older sibling, that the younger sibling is still developing or not quite there at the same level as her sister, and so on; whatever you make of it.
I mean, it's not laugh out loud humor, just connotations of words, but it's important to know that 美少女 definitely points to a younger type of beauty, like a good looking high schooler, while in this context they're reserving 美女 for the fully developed beauty of the older adult sister.
I guess the insertion of 少 creates a "nice save" moment, where he possibly avoids offending the younger sister by being quick and witty enough to save his previous statement which was intended to work as a compliment, and remains a compliment after the slight modification with 少.
It is not ironic. It may be slightly amusing/humorous since the user displays his wit in a clever way to possibly avoid an unpleasant or awkward situation.