When I look at pictures of Playmobil dioramas, I find the best ones include non-Playmobil scenery. Although PM rocks, grass, roads, trees, etc. do exist, sometimes a diorama can look a bit unfinished or incomplete if the creator uses only PM items. Just my opinion, but even Geobra use non-PM items in their promotional pictures and box art (this was especially true in the early days).
Perhaps the exception to this is when a creator produces indoor scenes (remember Toby's zoo visits and several other masterpieces by Raven?) which can look great with all-PM parts.