Little Mac does have a variety of weaknesses and strengths that make him suited for beginners:
Strengths:
-fast on ground
-tilts and specials have KO power, as do his very strong smashes
-smashes have armor
-has an OHKO move (not really, but close enough)
-all his grounded stuff has high damage output
-has reasonable approach options (somewhat predictable, but he certainly can get close easily), including a decent dash attack.
-probably other stuff I'm missing
Cons:
-bad recovery
-terrible air game
-due to the above, can't edgeguard offstage at all (at least I haven't seen any Little Mac ever pose a credible threat to any offstage character).
-poor follow-up ability to launching attacks
-some other stuff I'm sure I'm missing
Why does this make him very player-friendly to a new player? Well, among other things, I often see new players do a lot of things: randomly charge smash attacks, use tilts but not smashes or vice versa, jab a lot, run straight in and dash attack. Little Mac has the tools to make ALL of these things good - armor on smashes such that just timing the release means Mac almost always comes out ahead, his tilts are strong enough that smashes aren't terribly necessary (and vice versa), he has a very good jab combo (16% if you start and don't all extend the rapid jab), and his run is very fast (and somewhat low) which means others have less time to think and react to his approaches, plus his dash attack is none too shabby. He also has the KO punch, which is a single-use "Hit this and win or get a nice lead/reduce the lead" button - something that appeals to lots of new players, even if it's hard to land.
But his weaknesses make the picture even clearer. Oftentimes, I see new players do the following: not edgeguard, not be able to recover even when their character clearly can, not use aerials efficiently or at all, not follow up on launching attacks. The fact is, new players are not used to trying to follow up launching attacks or edgeguard, so they don't notice Little Mac's (relative) inability to do these thing. Since Little Mac is balanced around strengths almost all new players use and his weaknesses are things that many new players would not do on a character without them anyway, he seems very strong (which is also why many new players may think Jigglypuff is terrible - her air game and recovery (and rest) are fantastic, but comboing into rest is not easy for new players unused to comboing and many play so grounded that her great recovery and air game are simply unused).
This combination of factors leads newer players to label him OP and for others to label him "for n00bs" because he's in many ways built as a great character for new players, or people who would not use all the strengths of other characters. Thus someone who plays him may be labeled a new player, and thus bad at the game (though of course the label can be very inaccurate).
EDIT: Not all new players are the same, but I have seen far more who fit the picture I have described above very very well than those who are new and try to use aerials all the time or shield and grab a lot or whatever. This is why I paint new players in rather broad strokes - of course some break the mold, but far more seem to fit the mold very well.
And personally, I think Little Mac is a fine character to main, and I don't think someone who uses him is necessarily bad or whatever - I think he's a good deal of fun and an interesting challenge to use against characters with strong gimping and aerial games.
