We don't have any direct control over the appearance of PayPal's checkout site, but we have found that our Buy Now buttons for PayPal will bring up their mobile-friendly checkout.
However, the PayPal checkout button in our shopping cart still brings up the standard PayPal checkout because PayPal has not yet made their mobile-optimized checkout compatible with their Tax/Shipping Recalculation feature, which is necessary to make sure tax/shipping charges are accurate for the actual shipping address buyers provide during checkout. We've inquired with them about when we can expect their mobile checkout to support that, but we haven't received word back yet; I've just now followed up with them to seek any updates to our earlier inquiry on this matter.