It’s a bit of a complicated question and I think largely depends on your goals and what you want to get out of the challenges.
Personally I don’t think it’s cheating. I would say you should try to understand what it’s doing and why something works, but if an exploit is discovered and coded into metasploit what’s the point of trying to rediscover it? Seems overly complicated for some CTF machines.
I’m a programmer so I can read a lot of the metasploit scripts and basically understand what they’re doing. For me I see little point in trying to reinvent the wheel. If you read those scripts and didn’t understand them though maybe you should spend some more time trying to figure out why it works.
Another thing is, if metasploit is cheating what about nmap? Where does it stop? At the end of the day these are tools to assist you - the more you understand the concepts at a fundamental level the more you can leverage the tools.