Typically speaking characters can't morph into older versions of themselves unless their body is malleable like say a sponge.
And don't say something stupid like all cartoons are malleable just because of visual gags. Garfield is for all intents and purposes a pretty grounded world. It doesn't make any sense for him to just suddenly morph and then gain an aura that changes the laws of physics.
It's a forced meme reference, that's all it is. And you're free to enjoy it, but stop trying to pretend it's more than it is.
i’d argue the shine is plenty perfunctory - and i think its a neat bow on what is (imo) a pretty brilliant character design
the idea that garfield is a rushdown who has to be COERCED into doing rushdown techniques with treats + food is such a genius way to translate him into this setting
the joke (to me) is that playstyle being grafted onto garfield , a character so inherently lethargic and the means with which he accomplishes it w/ sugar rush. the fact he has a shine is just an element of that concept , so rlly whether it manifested as 70s garfield or not, there’s definitely vision behind it.
(( frankly though, if he had the pipe, i think that enigmatic aura that reflects projectiles would fit all the more. ))