In the third pic, it looks like you notched an inside corner. With aluminum especially, you want to wrap those corners and junctions with the strongest part of your welds. Aluminum is not as ductile or flexible as steel, and gets stress corroded with age. These junctions are where all of the stress builds up, and if you have notches or craters, they will crack and split apart. If your cart will be spending 99% of its life sitting around, it's not so much of an issue. But for things that will go over bumps or be twisted, lifted and such, I like to fill out those joints and sand or grind them down to fit or look good after the fact.
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