And selfies especially so.
Putting the camera too close to your face -- and arm's length is too close -- produces an image that diverges, in a predictable way, from how people typically see you. This divergence typically makes you look worse in the selfie -- it makes your nose look bigger, your chin weaker, your hair thinner, and your face longer than it appears to the eye in person.
Here's an illustration
(credit to Daniel Baker, with an explanation of the effect)
If you find yourself looking better in the mirror than in selfies, this perspective distortion may be part of the issue. First, you can stand further away from the mirror. Second, as you get closer to it, your two eyes spaced apart can stitch together a more complete view of your face than a single eye or lens could capture (try putting your face close to the mirror and closing one eye -- that side of your face gets abruptly thinner). There may even be some additional weird brain post-processing to make the perceived distortion less than it should really be -- kind of like how when you lay a coin on the table and look at it from an oblique angle, your eye still wants to tell you it's round.
(If you think you look better in the mirror but worse in all photos, it's probably just that you're accustomed to seeing your mirror-flipped face, so the unmirrored version feels wrong. I'd also conjecture that you could end up perceiving your face as being not just unfamiliar, but more asymmetrical than other people perceive it to be, because you've built your mirror-template in your mind and learned to ignore its asymmetry, until it's thrown into sharp relief by the unmirrored photo having the opposite asymmetry -- but the other people seeing you don't have this mirror-template of you to contrast against, so your asymmetry doesn't jump out so much to them.)
So what can you do if you want your selfies to better represent how other people see you?
Mirror selfie -- you can stand back from the mirror and get the distance you can't achieve with your arm.
Set up the camera/phone on a timer with a high image resolution, zoom it in (which might be partly cropping rather than true zoom, hence starting with high res) and stand four or five feet back from it.
Selfie stick?
Wait for this tech to become an app so you can do a virtual dolly zoom on your face.
Get a latest-gen smartphone? I think the latest multi-camera systems use some kind of black magic to partially correct the perspective distortion.
Failing that, don't base your judgement of your appearance, or solicit judgement from others, on selfies alone -- look at photos taken by another human from a reasonable distance -- better yet if they're a human with an actual camera and some knowledge of how to use it.
After all that, maybe you're still ugly -- but if you're asking here, you've probably gotten some mixed signals from various sources, and one of those sources might be your selfie camera telling you that you look like shit in a way that people IRL will not actually see. Don't be one of those people trying to get a nose job because they don't understand optics.
TL;DR: THE CAMERA IS TOO DAMN CLOSE