It is quite something to read some comments on how this must definitely hint at a “liberal” attitude and others replying with their own subjective impressions on how pretty it was to put the beach ball on top of the altar

The question here is: does Holy Roman Church have specific rules and regulations on this matter?
And our orthodox brother may not necessarily be satisfied with the answer “he’s the Pope, he does whatever he wants”, because - as Pope Francis time and again tries to remind us - he is the Servant of Servants above all else, so I doubt he would ever go with the “I can do whatever I want” mindset.
Usually you’d expect that only those things which pertain to the liturgy would be placed on the altar. Canon 1239.1 states:
An altar, whether fixed or movable, must be reserved for divine worship alone, to the absolute exclusion of any profane use.
What does this mean? According to The “New Commentary on the Code of Canon Law” by John P. Beal, p.1440:
Proper reverence for the altar means the only objects that may be placed on it are those which are necessary for the liturgical celebration.
Now the commentary seems to speak about the time of the liturgical celebration alone, while the Canon seems to refer more generally to the
purpose of divine worship - namely, restricting the possibility of placing something on the altar for the sake of it, for convenience, etc.
but not when it is a matter of divine worship.
In this case, the Holy Father had just returned from his first apostolic trip and decided to pay a visit to Our Lady Protectress of the Roman People, and he decided to place on the altar a special offering as a token of the love of the Brazilian people and of all the youth who gathered for WYD. Now the altar is a place for offerings to be placed before God, and in this case while it may strike as a sharp contrast, these usually profane objects had in fact the character of the sacred offering.
As a traditional Catholic, I am struck by the devotion in the prayer of the Holy Father, and the following phrase strongly resounds in my heart:
Let the little children come to me, and do not hinder them, for the Kingdom of Heaven belongs to ones like these.