No, Pichu/Pikachu are obtainable at the Trophy Garden as soon as you can access the route its located on. The only thing locked behind the post-game at the Trophy Garden are some of the daily Pokemon. If you're curious, the daily Pokemon locked behind the Trophy Garden are Jigglypuff, Igglybuff, Meowth, Eevee, Porygon, Plusle, Minun, and Castform. However, Pikachu and Pichu are not one of the daily Pokemon and are simply regular encounters in the area.
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