Is the McAfee virus alert on my iPad a legitimate warning?

Scam

See >>> Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

Do not respond to, or engage with, the message. Certainly do not, under any circumstances, interact with any -prompt to download or purchase an App. The messaged that you see is bait.

Most scam alerts are designed to scare the unwary into giving away sensitive information - or to fool you into doing something that you shouldn’t - usually to defraud you financially.

Providing that your iPad has been kept up-to-date with system software updates, you should not be overly concerned for your iPad being directly compromised. Due to the system architecture of iOS/iPadOS, unless jailbroken, your iPad is not susceptible to traditional malware infection per-se. However, as with all computer systems, there are still vulnerabilities and exploits to which you remain vulnerable. For older devices, no longer benefiting from regular security updates, the risk of an unpatched vulnerability being exploited increases. Regardless of the installed version of iPadOS, there are useful mitigations that can be used to significantly reduce your exposure to risk.

Threat Mitigation

The majority of threats to which you will be invariably exposed will surface via web pages or embedded links within email or other messaging platforms. Browser-based attacks can be largely and successfully mitigated by installing a good Content and Ad-blocking product. One of the most respected within the Apple App Store - designed for iPad, iPhone and Mac - is 1Blocker for Safari.

https://apps.apple.com/gb/app/1blocker-for-safari/id1365531024

1Blocker is highly configurable - and crucially does not rely upon an external proxy-service of dubious provenance, often utilised by so-called AntiVirus products intended for iOS/iPadOS. Instead, all processing by 1Blocker takes place on your device - and contrary to expectations, Safari will run faster and more efficiently.

Unwanted content is not simply filtered after download (a technique used by basic/inferior products), but instead undesirable embedded content is blocked from download. The 1Blocker product has also recently introduced its new “Firewall” functions - that are explicitly designed to block “trackers”. Being implemented at the network-layer, this additional protection works across all Apps. Recent updates to 1Blocker has introduced additional network extensions, extending protection to other Apps.

A further to improve protection from exploits is to use a security focussed DNS Service in preference to automatic DNS settings. This can either be set on a per-device basis in Settings, or can be set-up on your home Router - and in so doing extends the benefit of this specific protection to other devices on your local network. I suggest using one of the following DNS services - for which IPv4 and IPv6 server addresses are listed:

Quad9 (recommended)

9.9.9.9

149.112.112.112

2620:fe::fe

2620:fe::9

OpenDNS

208.67.222.222

208.67.220.220

2620:119:35::35

2620:119:53::53

Cloudflare

1.1.1.1

1.0.0.1

2606:4700:4700::1111

2606:4700:4700::1001

Security focused DNS providers intentionally "sink hole" known bad or malicious websites and resources - this providing an additional layer of protection beyond that provided by your device and its Operating System. These DNS services will, when used alongside 1Blocker or other reputable Content Blocker, provide defence in depth.

There are advanced techniques to further “harden” iOS/iPadOS (such as using DoH, DoT and DNSSEC). Apple has introduced its new Private Relay to its iCloud+ subscribers - in part employing ODoH (a variant of DoH) as an element of this new functionality. If you have subscribed to iCloud+, and have a device capable of running iOS/iPadOS 15.x or later, this feature is included. 

You might want to give this a read:

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

Any claims that pop up about your iPhone (or on your computer) being compromised while surfing the web are always going to be fake. No web site can scan your device for malware.

Regards.

The message is fake so don't continue pressing on the message. If you think about it, how is it mcafee knows you have a virus if you don't have their app installed? They don't, so that's the clue.