Yes, in most cases, "Guten Tag" is considered better and more natural than "Guten Nachmittag" when saying good afternoon in German (https://www.gradding.com/blog/german/good-afternoon-in-german). "Guten Tag" serves as a flexible greeting that spans from late morning until early evening. In contrast, "Guten Nachmittag," while accurate in translation, is seldom used by native German speakers.
It can come off as awkward or overly formal. Whether you're greeting someone in person, starting a call, or writing a message, "Guten Tag" is appropriate and well-received. If your goal is to speak German as it's actually used, prioritizing "Guten Tag" over "Guten Nachmittag" will help you sound more fluent and authentic.