The weather in Montreux is consistent enough throughout, so you can visit Montreux anytime of the year. Just keep in mind that the end of winter sees the temperature drop to below 'ZERO' digree; if you’re not accustomed to the winter, you might want to give this season a skip.
The Montreux is weather isn’t a determinant for a holiday as much as the crowd. Spring and fall is when you should ideally travel to Montreux as the tourists haven’t started descending on the place as yet. Between March and September is when you plan a trip here.