which may net you opinions from those who have gone through the process, the designers who may be in this group, and even laymen opinions (which may be useful for general perception).
As a career designer (who is not looking for work), I wanted to give some words of advice, especially since I see a lot of bad design advice in this forum.
1. Yes, design and graphics applications are more accessible than ever, and it gives you an opportunity to try things yourself. This creates a situation where more people can jump in, but also one that disregards process, theory or skill in design in favor of accessibility. TLDR; just because you can do some of it, doesn't mean you fully understand what you're doing and what effects it will have on a viewer. There are things often understood intuitively, like *general composition* (put logo at top or bottom, make it big) and legibility of fonts and such, but so much of what you learn from experience, DIYers can miss. The advice: be wary of knowing what you don't know; give yourself a **** awareness.
2. Those nuances can make a difference in viewer experience. People are looking for a reason not to read your book, just as much as reasons to! The advice: styles need to mesh and your cover needs to feel cohesive.*
Fonts have history, genres, and conscious/unconscious associations with them. Fonts from eras, whether they be the 30s, 80s, or now all say something about their subject. Once a font has been selected, there is also the art of typography to consider. Adjustments to sizes, positioning, thickness, separation from the background, color etc all work towards clarity while selling the tone AND setting of a story.* On this forum, I often see a greatly illustrated cover with a font slapped on, hoping it works out. This is art, without much design. Though solid 2D flat design is "in" in today's design world so it may still work, contextually it needs to mesh. Textures, 3D extrusions on text etc MAY be appropriate if it helps your cover feel complete. Look at some of your favorite covers and see what they do, how well a font complements the rest of the design.
Strategies of design have also changed over the decades as our tastes change, and create experiential associations as well. A big gradient or fluffy drop shadow often now feel outdated.* Strokes can be done well or look aged.* A design may intentionally evoke a famous designer to emulate a certain feeling, like say if you want something to feel retro and you design it like Saul Bass (cliche common example of a designer, sure, but you'll know him from the vertigo poster).
I've seen covers on here with accidental mixed media. One character painted one way, then another in a different medium or style that doesn't correlate.* "Good enough" situations for art/design. Make your media mesh together, including font! You have brushy art? Make your font match in tone or even execution. If they are intentionally incongruent, it better feel intentional.
There is art theory you only know if you learn it, and it's impossible to get it all into in a post. Composition, sure, where your eyes are drawn, but also color psychology, with how certain colors complement each other and attract attention or say certain things. Story wise, what emotions come into play with your color choices. Green harmony, orange alertness, energy, enthusiasm etc - they all have a subconscious effect that has also become attached to genres you need to be aware of.
TLDR: make everything match and know how it makes someone feel, or think about your book.
3. Your cover is important and deserves a ton of effort and respect! "Never judge a book by its cover" is nonsense because everyone does, and we can get so much information about a book from its cover. It is one of the biggest, if not THE biggest, marketing tools for your book. Polished design lends CREDIBILITY to your novel, just as with anything else that needs design. This effect is subconscious too. A reader may notice something is off with your cover, and think "this may not be worth the risk." Why spend all those days, months, years on your beloved novel to "hail Mary" it at the end with a 50 yard TD pass. Do it right, or hire someone who understands the nuances to do it right.
If some of this seems daunting, it's only scratching the surface. I'm not sure I've even been clear enough. You learn a lot of things about a craft when you do it for years or decades, just like anything else. Design can be a hobby, and is also a full-fledged career. That's not to say you can't do it. We are all creative, and you may find a new creative wave to ride, at a beach you were previously unfamiliar with. But arm yourself with knowledge, and knowledge of what you don't know, and take into account the level of experience of the people you get feedback from, just as you would a reader of your book.