No. Since computers can't convert to sentence case reliably, and since there are so many ways to import items into Zotero, including in bulk, we don't want to make it easy to end up with a library full of items with mangled proper nouns. Since you have to fix them manually anyway, we leave them as is and let you fix them when you're paying attention. (I assume you know you can right-click on the title to do the naive sentence-casing.)

On ZoteroBib we do automatically convert new items to sentence case if you choose APA, with an initial warning that proper nouns will have to be corrected. But that's a pretty different use case: you're specifically operating within a single citation style, and you're importing items individually in a consistent way, with an editable preview of each new item.