No, obtaining a PL-900 certification is not a requirement for PL-300 certification. Microsoft PL-900, also known as Microsoft Power Platform Fundamentals, introduces students to the fundamental concepts of the following Power Platform components at a very high level: Power Apps, Power Automate, and Power BI. On the other hand, PL-300 is more about advancing your skills in Power BI with an emphasis on data, including data analysis, transformation, modeling, visualization, and other related aspects.
The PL-900 is helpful for users who do not use the Power Platform frequently and those who would like to understand what the Power Platform is, but this is not necessary if the PL-300 is taken for the sole purpose of mastering Power BI. Suppose you are already experienced with data modeling and have some experience using Power BI. In that case, you may go straight into the PL-300 without embarking on the PL-900 first, as it is not a prerequisite.
For more information, you can check out the Study guide for Exam PL-300: Microsoft Power BI Data Analyst