Fill the entire house with turnips, how many do I need to buy? How many trips do I need to make to Daisy Mae

You probably want to take a look at this article: https://kotaku.com/animal-crossing-speedrun-shows-k-k-slider-isnt-that-pi-1842796083

Namely this excerpt:

It takes about 250 invisible “points” to reach three stars. Orcastraw outlines some of the criteria in a Pastebin:

The basic logic is as follows:

• Small non-crafted item: 2 Points(think flower in a vase, or a cardboard box, or a chair)
• Valuable non-crafted item: 3 points(the switch! it’s worth 30k bells)
• Basic crafted item: 3 Points(chairs, stools, birdbaths, anything that doesn’t require a lot of resources)
• Efforted crafted item: 4-5 Points(The apple tv requires 10 apples and gives 4 points, and the crafting table gives FIVE, so think that kinda ballpark)
• Public Works Project-escue items: 7 Points(The light pole gives this many points, and I imagine the other special nook miles-only items give that same amount, it’s a lot)
• Fences affect points also, in a roundabout way, with DECIMALS. 50 fences = 20 points, so do the math there? Idk

From the look of your blanket, I'd guess the problem is you're trying to fit too many rows/columns into a single tile - with the way ACNH patterns work, a 9x9 is going to lose a lot of definition and end up looking very blobby, as you've seen.

Square-looking picnic patterns exist, and most of them try to stick to around a 4x4 grid on any particular tile to make sure the pattern stays sharp when placed. See, for example:

(Source.)

As you can see, the creator sticks to relatively few squares per tile, and just repeats tiles to create the size they want. I would recommend not using the 3x3 pen tool directly to create your blanket, and instead using the tool to create larger squares.