First you have recognized that you're mediocre -- however you define that word -- which is good. It's the first step to better yourself. Imagine being mediocre and yet thinking you're hot shit. People like that are the most cringe and least deserving of respect imo.
Second, you don't need to become the best achiever or top-of-society-tier, you just have to become better than you were in the past. If you improve year after year, you may become outstanding before you know it.
Just don't fall into the pitfall of comparing yourself to others, as they most likely have their own issues you don't know of. Maybe that thriving CEO is suffering from an illness or aditiciton, or his son has just died. My point is, no one lives a life free of hangups and misfortune. I assure you not that many people have their shit together.
The thing is, struggle is inevitable in life, so you might as well struggle towards something that's worth it, like getting a better job, making more money, getting physically fit, approaching women, or just creating something of value. I know this is all cliché advice but it wouldn't be if it weren't true.
You said you drop the things you started, well that happens to many. I too have projects I failed at or haven't even started and it makes me disappointed with myself. We are bombarded with distractions all the time and society doesn't encourage us to pursue our dreams. You need a stronger sense of commitment and discpline to actually do what you want to do whether it's building a business, landing your dream job, or working on passion project. Time management is crucial for that -- you have to set "time blocks" either daily or several times a week where you work on whatever you want to work on and keep at it consistently for as long as it takes. The more progress you make the more you will feel encouraged to continue. Starting is the hardest part.
You say you're healthy and have time -- these are the most important things you can have in life so be thankful for it. Someone out there has it worse than you.
To answer your question, it's not that bad to consider yourself mediocre, just don't show that to other people as they will look down on you which will makes you feel even worse about yourself. So it's critical to have something that makes you confident around others, whether it's a good physique or a good job. I know these are materials things but sadly in today's world people don't really value values and morals so being a "good man" isn't enough to earn respect and may even have the opposite effect. People are more likely to respect you when you have something tangible to show, not like a car or a house, but appearance goes a long way, it's a reflection of who you are inside.
Lastly, criteria for success or mediocrity are subjective if you want to quantify them (like an annual income might be mediocre in one country but outstanding in another), but yes people are not the same.
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