Wishiwashi is generally first available once you reach Hulbury, with a 5% chance to fish one up in the town. It can come at Level 20 - 24, so its moveset will generally consist of Beat Up, Brine, Tearful Look, and Dive when you first catch it. Fortunately, it just barely passes the Schooling requirements when you catch it, so you can instantly have a very powerful Water type on your team -- Dive especially will be doing quite a bit of damage. However, while Wishiwashi is serviceable, there are several reasons you shouldn't use it in your playthrough.
1. Gyarados
Magikarp is available earlier, and though Gyarados doesn't have the same powerful stats as School Form Wishiwashi, it also doesn't have the weak stats of Solo Form Wishiwashi. This explanation holds true for a lot of other Water types, chief among them being Drednaw, Seismitoad, and Inteleon. What sets Gyarados apart, however, is how effective it is in-game. At Level 21, it gets the immensely powerful Waterfall, followed immediately by Crunch at Level 24. Using the Move Relearner can get it Ice Fang, and a bit later down the road, it also gets Dragon Dance. All this together makes for the perfect combination that can tear up most threats in the game, especially because Galar doesn't have any noteworthy Electric specialists.
Gyarados simply has a better movepool than Wishiwashi and also doesn't have the drawback of Solo Form, making it the better pick in most cases. If you get the choice, Gyarados is far superior.
2. Solo Form
This was briefly touched upon in the last point, but while School Form Wishiwashi is a force to be reckoned with, its Solo counterpart is anything but. It's unable to do much against most opponents, and if your Wishiwashi can't OHKO an opponent, it has a very strong chance of becoming deadweight for the rest of the battle. Galar is easy enough to navigate around this with healing items and switching, but the mere fact that this has to be done is a point against Wishiwashi.
3. Its Speed
Wishiwashi is very, very slow. Speed doesn't matter as much as it does in competitive, but it can still mean the difference between needing to take a hit or not. Some slow Pokemon like Toxapex can pull it off due to their monstrous defensive stats, but Wishiwashi also has very low HP -- not even its massive defenses can support that much. The Speed stat of a Pokemon is always important to consider, and it may be more optimal to simply choose a different Pokemon.
Now, this isn't to say Wishiwashi is a bad Pokemon. Its attacking and defensive stats are all incredible, and it does get good moves like Liquidation, Earthquake, and U-turn. It can be caught early and be an immense asset to your team, if you play it correctly -- it just takes more skill and matchup dependency to use it well.
TL;DR Wishiwashi is okay, but Pokemon like Gyarados are better due to their better availability, better movesets, and less choosy matchup needs. Wishiwashi works, and it can do well if you play around its flaws, but it's generally less efficient than using Gyarados.
