I’m not quite sure why you’re telling me this but I will humour it and give my two cents. Because, you see, I do truly believe he is an incredible actor.
I think when someone rises to popularity with comedy, it’s a hard thing for anyone to look past and see that he is in fact talented - and that his entire career isn’t just built up of the crazy facial expressions, over-acted gestures, and funny catchphrases that everyone associates him with. There are many performances he has given that have more than proved how capable he is at acting. (The Majestic, The Truman Show, Eternal Sunshine, The Number 23, A Christmas Carol, etc.) Hell, there are even funny movies he has done that showcase just how talented an actor he is. (Man On The Moon, I Love You Phillip Morris). I personally think if what is stated above is how you feel about him and you want to think he was a better actor, then you’re not looking in the right places or you’re not looking hard enough.
On another note, even in some of his comedy movies his acting can be pretty fantastic. It’s all right there in front of you, it’s just hard to see because you’re too busy laughing. Comedy itself is a truly hard thing to pull off - so if he is in fact making you laugh, which is one of the hardest reactions to garner from an audience (considering everyone has different sense of humour) then clearly he must be incredibly talented at acting in comedic circumstances in order for you to find him as funny as you do. Ergo, a great actor. He might have a specific way of getting a laugh out of an audience - but it’s not like anything anyone else really does.
Try this: next time you’re watching a Jim Carrey movie, stop and think to yourself just what he is doing to get the reaction he is getting from you. (I like to use The Grinch as an example, because when I’m watching that movie - as ridiculous as some of the stuff is - I always forget even if just for a moment that that is actually a man in The Grinch costume and it blows my mind every time, because no one else would act the way Jim Carrey does in that movie.) And if you still don’t think he’s as good an actor as he really is, then you probably won’t ever be able to see it and I can’t help you with that.
It is all a matter of opinion of course. Some people don’t even find him funny, and hey, to each their own.