I always start matches by keeping an eye on my opponent's movement, and I try to see if I can find a pattern in their playstyle that I can exploit. This is something that I do even when I face good players, but it is extremely helpful for dealing with roll spammers.
Once I've decided that my opponent is using rolls way too often, I'll concentrate on trying to bait out rolls and seeing how they tend to react. As a result, it is very easy for me to spot habits that I doubt my opponent even knows they have.
The two most common habits I find in roll spammers are that they usually roll away from me when I dash towards them, which I can punish by delaying the time I press the grab button, and rolling towards me when I stand a certain distance from them, which I can punish with either a grab or a smash attack. The point is, when dealing with those kinds of players, you often just want to play more passively than usual and let them dig their own grave.