No, the encounter table for each area in the game is usually fixed. (for example, the one for route 201 is full of Bidoof and Starly). It can vary depending on things like time of day and, on certain routes, whether or not you have a GBA game in the slot. In this case, getting a Pokemon put into the Trophy Garden as Pokemon of the day modifies the encounter table for the Trophy Garden until that Pokemon no longer appears. The first Pokemon you get put in always goes in the 7th slot. If you get another Pokemon added, the new Pokemon goes in the 7th slot and the first Pokemon moves to the 8th. All encounters in the Trophy Garden use the same encounter table.
Basically, once Marill is put in, it is always in the 7th slot, unless you get something else put in as well (the next Pokemon of the day), in which case Marill moves to the 8th.