If you are trading for small pip gains off of 4 H and lower timeframes, ya sunday is too slow. Not a lot of traders trading, so not a lot of movment, which means you can get some very unpredictable movement. Or, price can just sit there for hours and hours doing almost nothing. So, it can be like watching paint dry.
However, if you trade daily candles and above sunday can be a good day to place trades if you intend to be in them for most of the week.