Everyone here has given some really good advice! I think it just sounds like your knowledge of fashion and clothes is very small. Once you understand all the different types of clothing that really is out there, I think your brain will be able to come up with a lot of fashion ideas much easier! Let me tell you, Pinterest is a life saver. I have at least 5 different boards full of clothing and character ideas, both drawings and real life references! Its always nice to have to give you some inspiration!
Character design is a lot more than just hairstyle and clothing though. It's height, posture, expressions, body type, hand size, soft vs. sharp lines, shiny vs. matte, the list goes on. I think another issue you have is that your art skills are very limited, so practicing anatomy and studying different body types will help you a lot as well! It's not as fun as designing the characters, but it really does make a difference.
Now im definitely no expert, far from it lol, but character design doesn't have to be super exaggerated and out there! Characters can be simple and still be memorable. Take these sketches of 4 different girls I made for another comic Idea I have for example. Their designs are definitely not finished, but i'd say they're already not too bad with what I have already and they're far from anything "extravagant".
There's a couple reasons for why they're already pretty interesting in their very early designs.
1- Shape. I've incorporated lots of shapes into their designs! The first girl has a rounder, flowy, and soft design. Her hair is curved, her pants and shirt are flowing, and she even has some actual circles in her design (like her hair beads) Shes 14 and just a very cute and wholesome character so I wanted to portray that in her design. Contrast her with the girl on the far right. Her design is much more sharp and angular, even portrayed in her pose. I wanted her to have a more authorative vibe and more strict. You can also combine shapes in characters! The 2nd girl from the right has a design that uses a combination of curves and triangles, especially visible in her hair. Shapes are a HUGE factor in vibes a character might give off.
2- Layers. Layers in clothing adds more variety and makes things more appealing to look at. All of the girls have multiple layers of clothing on and this already makes their designs much more interesting!
3- Pose. The poses the girls are in tell you basically all you need to know about their core character. It might not cross peoples minds when they're designing a character, but poses and body language are HUGE in conveying things about the character.
There are a lot of things I could change or add to make them even more visually appealing. Accessories, larger variety in body type, and color, for example. Watch what happens when I added color to one of the characters designs-
okay shes much cooler now lol, and the use of warm colors helps invoke her firey and fun personality! Even with my pretty basic anime-styled art, I can create some pretty interesting character designs! I gave her a katana as shes a trained assasin, plus it matches her japanese inspired outfit! Little things can be very telling!
Okay, this is all fine and dandy, but what if your character is, well, normal lol. Like not a fantasy character, but just a normal guy? You can still make some pretty interesting designs! One thing I like to do personally, and its not required obviously, is to give each character 1 staple piece of their design. Perhaps its a hair clip, maybe its their hair itself, or a scarf, maybe its the big scar across their face! It can be something little or something really big! I tend to make the hair on my female characters their defining design point and skin details on men (not sure why lol). All it takes is one simple accessory or pattern to spice up a character.
Take one of my characters Silas for example. I'd say his staple design choice would havbe to be his glasses and turtlenecks. Is he the first character to ever do this? No. Is he the most noteworthy character to ever do this? Definitely not lol. But if I compare him to the REST of the characters in my story? He definitely stands out. No one else wears glasses and hes probably the only person that wears turtlenecks in the summer lol. Your character doesn't have to compete with the rest of characters in existance. Just make them stand out compared to every other character in your story!
Another one of my characters, Melody, loves to wear girly things and has emerald green eyes. My other character, Calix, has a weird ring he wears everywhere and messy wild hair. Jayden, has a blue stripe right through his bangs and the same shade of blue underneath his hair, Layla wears very pastel and soft colors all the time and has cute space buns, Lance has a man bun and a scar right across the bridge of his nose from eye to eye. These are all just simple choices that make them stand out compared to the others and tells the audience a bit about who their are or who they might be!
That I think is the most important part about a character design. The audience should be able to guage a bit about them just by a single still image of them, what they're wearing, how they stand, and their defining shapes and colors.
Taking your character thats in or was in the military. Perhaps he wears a dog tag, maybe hes got a buzz cut, wears clothes with holes or ripped and tattered, dark greens and browns, etc. But you can go beyond just where he was in life, what's his personality like? Is he a messy person? Maybe he leaves his shirt half buttoned because he just doesn't care to button it to the top and never brushes his hair. Perhaps he has some tattoos that are reminiscent of things important to him. Is he cold and brooding? Maybe he likes to wear dark colors cuz it reflects his soul (lol) and constantly wears hoodies because he can hide in them from the rest of the world.
Think of your characters personality and find ways to reflect that in their design!
I appologize for the long post lol, I had a lot to say xD