Quality of English in a college recommendation letter
The people reading a recommendation letter are interested in the following two things:


What is the assessment that the recommender makes of your skills and potential?
How much should they trust the recommender's assessment?


The style and errors of the recommender primarily speak to the second point.  It is hard to see how an occasional "normal" mistake will make a significant difference in how your or the letter-writer are perceived.  Likewise, if the writer is not a native English speaker, but makes mild typical grammatical errors that do not impede the sense of the document, it is just part of the normal "friction" of cross-cultural scientific communication and should not be a problem.

If, however, the mistakes are frequent and significant, then it may undermine the credibility of the recommendation.  Frequent or severe mistakes can make a reader think either that the recommender doesn't really care about this letter or that they are sloppy and perhaps don't have very good judgement to begin with.  Fairly or not, this can also apply to foreign-language issues, since a professor who knows their English is not good can get proof-reading help from any number of sources.