I have some friends who are ridiculously offensive when they play. No mindgames or neutral stance or anything. They just sorta run up to you and do whatever. Is this something I should expect in real tournaments and how should I deal with it?
The first thing you should know is that counter is actually really good against this kind of player. If they always just run in and attack with no variation, it'll shut them down and force them to slow down and insert some mind games or grabs.
The second thing is that wavedash back into D-tilt or F-smash will beat anyone who just runs in against you. Same thing with dash dance backwards into grab, it'll beat someone who attacks the space where you just were. If they're being predictable in their approach, that'll blow them up in neutral.
Thirdly, don't be afraid to shield and roll or wavedash away. Yes, it's bad if you become predictable with it, but you shouldn't just ignore your defensive options.
However, just because someone is being all-out aggressive does not necessarily mean they're playing badly. Mango is the perfect example of this, as he goes in nearly every chance he gets, but he's still one of the best players around. If you're good enough at it, you actually can just run up to people and nair-doubleshine-grab or such.
Just remember, as long as you can move well enough to retreat and hit them with your sword (or grab them) when they come in, you'll win the matchup. If they can outmaneuver you and come in when your'e not prepared to deal with pressure, they'll win.