The Supreme Court justices have often been active in the selection of new justices. Do you think this is appropriate?

Assuming you mean the Supreme Court's calls in the past year to fill the vacancy left by the death of Justice Scalia, then yes, it is appropriate because the role of the Supreme Court is to uphold the US Constitution and the laws of the United States against all who violate them, including the legislature.
Article III of the Constitution created the Supreme Court of the United States. Since then, laws passed by Congress have set the number of justices that are to sit on the Court.
However, when Justice Scalia died and President Obama (of the Democratic Party) nominated his replacement, Merrick Garland, the Republican-controlled Congress refused to partake in the nomination process. Instead, they let the clock tick out until President Obama left office—a period of 293 days—so a Republican could become president and nominate a different Supreme Court justice.
During this time, several sitting Supreme Court justices voiced displeasure with the political maneuver to pack the Supreme Court with votes that were meant to be friendly to one political party. These sitting justices were active in criticizing Congress and urged them to uphold the law and select a new member of the Supreme Court.