This is probably most often a Vanitas/ Momento Mori type of thing. The basic idea being that the recognition of one's own mortality before the infinite existence of God is a pious act of humility. It was also something that was very much on people's mind in Medieval and Early Modern times as sickness and violence were very common and everyone was that bit closer to suffering and death, even the rich. The Black Death and the beginning of the Little Ice Age in the 14th and 15th century in the Late Medieval period caused a notable upsurge in morbidity because both events cause visible and widespread worsening of conditions, nature's Momento Mori essentially.

One interesting example of personal interest to me is the tomb of Robert Cecil, 1st Earl of Salisbury which is a late example of the marble skeleton design that was more Late Medieval but this is early 17th century. The rest of the tomb is colourful and even includes gilt elements, spared no expense, but the actual monument is comparatively humble, including no extolling of his public career but emphasising the four cardinal virtues and with the obvious skeleton element. As Pauline Croft in "The Religion of Robert Cecil" (1991) says this part is stoic. The connection between the cardinal virtues and the skeleton in term of stoic Momento Mori religiosity is obvious, especially when combined with the emphasis on biblical images in the decorations around it. You can see it as the idea of a humble man before the glory of God which surrounds him. The tomb entire obviously does glorify the dedicatee and the idea of mortal Christian virtue is kind of a humblebrag, but you get the idea.

in the case of Montjuic Cemetery, since it was founded in the 19th century, it would be to do with the 19th century fad for the macabre and morbid as well. There was in interest in this because of the growth of large cemeteries to host the urban population output, which made death more visible, as well as other cultural factors like urban epidemics. But this would also tie into the older Momento Mori tradition. Heck, the Momento Mori concept stems from the Ancient Greek Stoics somewhat, so it's a rather enduring concept in Western culture.