If you play around with advance multi table options on stars you can make it easier for youself. I left click and assign colour. bright blue for abso re-tarded players, easy to see them when multitabling, also Red for very good players who have made lots of money. Lots of other colours for everyone else inbetween, but for the stakes i play i really only want to know the idiots and the good players. Quite surprising online even at the low stakes MTTS ive come across some really sick grinders making 100s of kkkss.
I just write quick things like if they OL (open limp) and call raises OOP (out of pos) or if they are tricky like OL and then 3 bet. What size they open raise, nowadays anyone online who open raises more than 2.5x with a stack of more than 20bbs is usually inexperianced (IMO anyway).
Another good way of telling a little about them, is find out how many tables they are playing, make a note if they are playing more than 6 tables, look to see how many stars they are advertising, is their avatar a picture of their kid/ GF/ pet or themselves usually means they are not so good. Oh and also country is a pretty big help. For example ive found out these last few months that russians are usually hyper agressive, etc. you can tell alot about a player without ever seeing a showdown.
If the hands go to showdown, i just make a quick note of how they played their draws, weather they fire 2 barelles and give up, or if they check rivers oop with SDV (show down value) to induce a bet. Also how tight they play from the Blinds or LP, and around bubbles too.
Not much point in taking notes on standard play, but if you see anything funky, like they open raise then fold to 3 bet shove getting 2-1, then i would make a note of this kind of thing.