Technically yes. But when you connect to the server the browser is also sending the address in plain-text (see SNI).
This will eventually be fixed with an extension to TLS 1.3 (see ESNI), but right now every address you access is sent via plain-text, thus can be intercepted by ISP unless you're on a VPN.
That said, PiHole's main selling point isn't that, it's blocking ad domains and for that it's doing a might fine job (I've had it running in my home for 6+ years).
