I can't say for sure if Nina Dobrev is ticklish or not, but you could try asking her on her ask.fm page!
She is very active on there, answering 7251 questions, so who knows, you might get lucky.
If you're a Nina fan, there's a really long Q and A video you might like where maybe she'll answer some other things you might be interested in:
To speculate at your original question though- she probably is ticklish. In a 2008 study conducted on students, only 27% claimed they were not ticklish at all, with 89% admitting they were ticklish on their right foot, 80% under their arm, and 77% on their left foot and ribs. So it really is a very common affliction!