If your streusel topping becomes pasty (or even batter-like), chances are it's from adding butter that is too hot, or (less likely) over-mixing. To remedy this, allow the crumble to cool completely, then use a fork to break it up and add a tablespoon or two of flour to absorb the released fats. Note that nut crumble topping is less likely to turn to paste because the nuts absorb and temper the butter, but it happens.