Sure you can front static assets with the free Cloudflare tier...
[a] which is fine if you're happy with the inflexibility that free Cloudflare offers. And you live in a country where the free tier doesn't have horrible routing (eg use the Sydney AWS region, put Cloudflare in front of it and then watch your traffic to/from Sydney take a round trip via the US or Singapore)
[b] every single AWS service charges egress fees (ie Cloudfront doesn't help at all)