Eh, I'm going to take the unpopular route and say yes she can get stronger based off a few non canon sources. Gero in GT states that 17 was originally suppose to be stronger than Cell, but he was too unpredictable and had to be restricted. Secondly, 18 in Yo! Could read ki when before she could not.
I'm not sure that 18 is specifically stronger in the Boo saga, but I think she certainly has the potential to increase.