Whether a letter is considered a common letter in a language can depend on various factors such as its frequency of use in that language, its historical and cultural significance, and its prevalence in loanwords from other languages.
In the case of the letter "J" in English, it is not one of the most commonly used letters in the language, but it is still a recognizable letter and is used in many English words. Additionally, as you mentioned, it is often used as the first letter of popular names in English, which can contribute to its familiarity and perceived commonness.
However, the fact that "J" is also commonly found in loanwords from other languages does not necessarily make it a common letter in English. Loanwords are not native to the language and may not reflect the frequency of use of letters in the English language itself.
Ultimately, whether a letter is considered a common letter in a language depends on various factors, and the use of loanwords is just one of them.