not rolling R's is completely different from a Boston accent not pronouncing them at the ends of words (non-rhoticity). a rolling R is when u make a buzzing sound with the tip of your tongue, and it's a common sound in Spanish. when someone can't roll the R, they're just making a regular R sound, and it's how a stereotypical American would sound trying to pronounce the Spanish word 'perro' (dog).

a better analogy with the letter R in English would be the way some 3 year olds would say 'wobber' instead of robber. they're not yet able to pronounce the R sound, and it would be the equivalent of not being able to make a rolling R in the languages that use it.