Karrablast is not very viable in Gen 8 LC. It has the unfortunate situation of being a Bug type, and Bug types aren't too great in LC. One of the most popular Pokemon at the present is Vullaby, who can easily smash through Karrablast. Archen, Pawniard, Mienfoo, Onix, and other common Pokemon can all easily deal with it as well. Archen is faster, and can OHKO with Head Smash or Dual Wingbeat. If Pawniard sets up a Swords Dance, it's all over for Karrablast. If it doesn't, Karrablast still isn't lasting long. Mienfoo is more of an annoyance, but with strong, Life Orb-boosted attacks, even it can take out Karrablast. Onix can either use Stone Edge right off the bat, or Dragon Dance + Berry Juice then Stone Edge, leaving Karrablast in the dust either way. Stealth Rock is also extremely common, and guess what this little bug is weak to? Using Heavy-Duty Boots means you can't use a Choice Scarf, or a Life Orb, or something to make Karrablast better.
Karrablast is outsped by quite a few Pokemon in the tier, and it's frail, even with Eviolite, which is not a good combination. This leaves it susceptible to being eliminated from the field within moments (looking at you, Archen), perhaps not even getting to do a thing.
It faces competition from other Bug types as well. Dewpider is a better lead, with Sticky Web and it being more defensive. Anorith is faster, has a higher Attack stat, and gets Swift Swim, making it viable on the few weather teams in LC. Almost every other Bug type is bad/not worth mentioning, and Karrablast joins the ranks of the forgotten.
Bug also is not a very good offensive type, only hitting a few Pokemon in the tier hard. One of the more notable instances is Grookey, as Karrablast can (usually) live a Grassy Glide and return with a Megahorn. It can also be pretty good against Abra, but beware of the occasional Sash + Counter tactic in employs. Some of the more common Pokemon in the tier tank Bug moves easily. Mareanie isn't bothered by Megahorn at all, and can shrug off the damage with Recover and Regenerator. Common Fighting types like Timburr and Mienfoo can also take a Megahorn, and so can Magnemite, Foongus, Spritzee, etc. While Karrablast does have decent coverage options, they're still fairly weak, and if they aren't (Drill Run, Poison Jab, maybe Knock Off), Karrablast is often too slow to use them too well.
This isn't to say Karrablast is awful, per se. It can certainly do damage, and a lot of it to an unprepared team. The trouble is that it's too slow, outclassed by other Bug types, and its weaknesses are easily exploitable by many other Pokemon in the tier. I've seen Karrablast in LC a few times, but not one of them has lived more than one attack.
Hope I helped!