Is avast.com a legitimate fix for iPhones?

NO!! No iPhone needs any third-party "protection" help. The sandboxing systems built into iOS make any infestations impossible in an as-shipped iPhone or iPad.

Don't fall for fear marketing.

No, as well as any other app that claims to "monitor", "protect", "optimize", etc your phone.

iPhone security is the best in the industry. There is nothing you can add to “improve” what is already almost perfect.