Firstly, the humor or irony in the sentence lies mainly in societal perceptions around beauty. When the term ‘ugly’ is used, it suggests that the girl doesn't fit within the conventional standards of beauty. However, the irony here is that she has enough self-esteem to place herself in the middle of every picture, signaling confidence that is typically associated with more traditionally attractive people. This reversal of expectation is what makes the situation funny because it contradicts stereotype.
Secondly, the girlfriend displaying assertiveness in situating herself at the center of every photo can be humorous for those who understand this action as an attempt to occupy a visually prominent position despite not being traditionally attractive. Most people, when they lack confidence on their looks, would prefer to stay at the back or at the sides in pictures to avoid attention. In this situation, the standard order is being willingly mixed up, creating an unexpected amusing outcome.
Thirdly, the irony also comes from the way society tends to value beauty. There is often an expectation, conscious or unconscious, that 'beautiful' people should always be at the center of attention, especially in photos. Therefore, when the 'ugly' girlfriend insists on putting herself in an attention-grabbing position, it subverts commonly held assumptions and values related to aesthetics.
Fourthly, this sentence can be seen as a critique on society's obsession with beauty. The humor lies in the girlfriend's disregard for societal pressures and norms. She might be aware that she doesn't conform to conventional beauty standards but doesn't care because she embraces her unique characteristics. This action becomes a comical jab at the societal beauty narrative.
Fifthly, there could also be some self-deprecating humor discernable from the phrase. This may be particularly true if it's actually the girlfriend herself who describes the situation or uses the term 'ugly' in a kind of self-parody. This would demonstrate a degree of self-awareness mixed with a refusal to comply with the traditional rules of appearance, which can be quite hilarious.
Finally, the irony is not just on the fact that the 'ugly' girlfriend insists on being in the middle, but also on our perception of 'ugliness'. Many people would find this kind of self-confidence and assertiveness attractive and admirable, therefore challenging our narrow definitions and expectations of beauty, and hence causing the humor in the situation. When the ugly girlfriend insists on being in the middle of every pic