No, the choice of letter case generally does not significantly impact the likelihood of successfully registering a trademark.
In rare instances, the resemblance between certain letters in specific typefaces (such as the lowercase 'l' and the uppercase 'I') might introduce an additional layer of interpretation that could potentially work in favour of the trademark applicant. However, these cases are infrequent and exceptional. The choice of letter case is considered a mere stylisation, and because of that, unable to significantly influence the overall distinctiveness of the mark.