Okay, so a number of things that pop out here:
The cover and banner look like they're different genres. The cover looks like a comedy harem romance, while the banner looks more like it's more of a "battle manga" with a very different art style. While it's not a bad tactic to try to emphasise certain elements of a series with your promo materials on a series that has a bunch of stuff going on, it's probably best to be consistent with which element you emphasise on two things that'll be seen together like this. So if you feel like selling it as a romance will do better on Tapas, make it look like a Romance here, and keep assets that make it look more like a an action story to apply consistently on a platform like say Comicfury, or for ads somewhere like Reddit.
And yeah, it's totally acceptable to just use part of your cover for the banner. It's mostly only even seen on desktop and the forums and not worth commissioning or making a bunch of new art for.
The font on "THAT" and "ISEKAI" is... not a great font. It's very thin and doesn't read well when it's so small on the cover, and it's not a font that clearly contrasts the "Stick Figure" part in the middle. I'd recommend experimenting with maybe a more "Fantasy Comic" font vibe, like really ham it up, go with shiny text and a fantasy font and stuff like an isekai comic logo (look at popular ones for inspiration)... and then have this scrappy "Stick Figure" on lined paper tearing through the middle to really sell that concept of the comedy that it's an isekai... about stick figures.
Finally... the cover itself. It's not really using the space as well as it could, and there are issues with the readability, so making a new one is probably a good idea. The marker texture on the sofa, while nice, is dark and busy and makes it hard to read the shapes of the stick people. The "human" form of the protagonist isn't clearly related to the stick figure one (not directly above, in the same pose or similar), it looks like he's a different guy skulking behind the sofa. The flat purple is may be not as interesting as surrounding detail as it could be.
I think if I was to try to design a cover for a series like this, I'd want to really try to communicate the concept of "It's a handsome protagonist guy who's been sucked into a stick figure world with a doodle aesthetic" as clearly as possible. Maybe do something fun like... I dunno, there are fancy flowers and sparkles like a romance cover on one side, but they transition into doodled flowers and stars on a lined paper background on the other, like really milk that juxtaposition of "handsome protagonist with a modern fantasy comics aesthetic" and "stick people doodled on lined paper" for comedy and the high concept of the series. Or maybe have him falling into the doodle dimension, or have a romance looking cover part torn away revealing the doodle style beneath. See how much you can really push it to sell the unique concept of the series.
