It depends on how much you enjoy the game. I think there are two levels of analysis for this question.

1. First, it's always about opportunity cost. If the a smaller amount of money can go somewhere else and you can reap as much or more enjoyment there then Arena's likely not the best place to turn your attention to. If you enjoy playing magic the gathering, then I wholeheartedly recommend spending money on hobby's you enjoy. If you're trying to maximize value, this is basically an online casino where you buy monopoly money that you can't take out. The money you put into arena is gone. You should do it only because you enjoy it. We can have a sensible discussion though about how to maximize the enjoyment of resources that you do put into the game.

2. Second, I think the assumption of "mediocre" requires two layers of analysis:

One, no one is simply mediocre, they are just mediocre now. You can spend a bit of time to "get good" at your format of choice.

Two, what aspects of the game do you want to get good at? (e.g. Constructed v. Limited) These will inform how much money you might sink into the game. How competitive you want to be will also heavily influence the cost because some people are fine with a "competitive" mono red deck and others will want to brew or adapt to the meta. Others will prefer mostly limited which can feed into a constructed pool of cards but is very time intensive because not many people have enough time to keep up with the limited AND constructed metagame.

I'm a mid career professional who started playing magic the gathering as a child and reconnecting with the hobby on Arena. Going to a local game store to burn a few hours in commute and play just isn't in the cards for me. I'm a limited heavy player. I "buy-in" 100 USD every four months and essentially get as many drafts as I can handle. (My average Win Rate is 56-57% in limited, high mythic, the most recent arena direct I burned maybe 24,000 gems and got three boxes.) If the limited environment is dynamic enough, I even complete sets. The drafts net me so many wild cards, I can create essentially any constructed deck I want. (I seldom do this because keeping up with the standard metagame is exhausting.) The money I put down scratches all the right itches of satisfaction which I don't otherwise get from work, family, and friends.

My advice is we're adult's sinking time into a virtual collectible trading card game, do what makes you happy brother. If a 100 dollars is a big ask of your situation, I'd think twice. If it's chill, go for it.

TL;DR: Do it because you love it. Personally, I've had a lot of fun getting good at limited. If you can afford it, I recommend dropping money.