I just put my hair into a bun and wear sweats. Its too much effort with little payoff to do all of the senseless beauty regimen stuff that most girls do. I feel better simply not giving a f***. I feel free an I feel like I'm staying true to myself. I value my dignity more than meaningless attention from guys. Okay I typed a wall of text about how arbitrary stuff like dating and short term relationships with 'boyfriends' are but its pointless
Being plain is actually unique. What he means is he wants you to wear the uniform of the attention starved. Go take a look at Facebook and instagram profiles of girls who take loads of pictures of themselves in different angles and outfits to get an idea of what I mean.
Actually I'll just tell you what will probably help you get what you want. Most likely what you'll have to do is straighten your hair, go heavier on the makeup, and wear tight and revealing clothing (see forever21, urban outfitters, h&m etc). If your hair is only shoulder length or shorter you probably need extensions because guys really like very long hair. On lazy days wear yoga pants or leggings as pants with no underwear under to emphasize your ass crack and allow maximum jiggle. On dates make sure you're very teasing and flirty. Don't act masculine even if you naturally have a masculine personality. Be seductive and suggestive. Also become society's definition of 'fun'. Laugh at sex jokes and have a few of your own. Some guys and girls will say girls who are like that are trashy but notice they always have boyfriends.
I think I saw you mention this guy in another question. He's a life counselor, right? I'd really need to see a picture to see what he's talking about. Plain jane can mean a lot of different things to different people, it can mean that you don't put an effort into your appearance, but it can also mean that you look average and don't stand out in a crowd, or it can mean that you're a girl next door type (a girl who is cute, but isn't a traditional beauty).
Some guys love it when a girl looks really natural and doesn't wear much makeup and some guys like it when girls wear a lot of makeup(i'm talking caked on and tons of eyeliner) and some guys are just douche bag perfectionist. I think this guy falls in the last category and should be avoided. Let me elaborate on this. He is the type of guy that actually goes out of his way to notice flaws in someone and a lot of it probably has to do with his own insecurities. Do not even use this guy as any real life reference. Seriously. He is looking for an insecure girl so that he can feel like he is in control of the relationship and that will bend over backwards to meet his unrealistic expectations. When you look in the mirror and actually think, "damn, I look really good" that is the look you should always go for. If you throw on more makeup. and don't feel comfortable with the way you look then your uneasiness will translate into you not feeling confident. If you aren't into wearing tons of makeup and just like the more natural look you need to find a guy that feels the same way. Don't be the girl that is afraid to stay over at a guys place because you don't want him to see what you look like in the morning without makeup.
I think you're trying to get the approval of the wrong guy. He is only one guy and does not speak for every guy on the internet. He may also just be lying to you because he finds pleasure some how in making you feel insecure. Lots of people on the internet will do that. Sounds like he goes for the Kim Kardashian types if he's suggesting that you don't stand out.