Supreme Court appointments are critically important, but lower court appointments are important, too.
The Supreme Court is one court of nine people. It cannot and does not correct every wrong decision rendered by lower courts. Not even close. The high court takes about 1% of the cases it is asked to take. It takes a higher percentage when the Government is asking, but not all.
Bad appointments to lower federal courts can have very long-lasting effects. The Ninth Circuit was expanded during Jimmy Carter's single term. The appointments he made, no doubt strongly influenced by California Senator Alan Cranston, produced the notorious "Ninth Circus" that plagued the Far West for an entire generation.
One of those appointees is reported to have said, regarding the Supreme Court and Ninth Circuit decisions, "They can't reverse them all." Whether he really said that and whether, if so, he was joking (as he claims) is beside the point. It is undeniably true. The Notorious Ninth commits far more errors than the Supreme Court can possibly correct, and it does so knowing that many of these decisions are contrary to what Supreme Court precedents actually require. Even when they are eventually reversed, those errors do damage in the interim, and the interim can be a long time.
During the Bush 43 Administration, the judge-pickers had a motto of "no more Souters" for the Supreme Court, but they did not apply that principle to the lower courts. They made some solid picks, but they made some that cannot be explained on any basis other than politics and personal connections.