Yes, it's the same general joke as an earlier comic. I happen to like the classic triplicate buildup joke, sometimes extended to the quadruplicate buildup.

Now, one may ask exactly why Alex is throwing up a spell in the last panel as she patiently mocks her brother. The spell, of course, is nothing more than modified and slightly focused direct force, one of the base spells of any sort of magic movement (Besides collateral movement created by things like a large explosion, for example). Such spells are commonly used to move largish objects from place to place without the need for brute force or mechanical wizardry, such as in excavation, building, and other such tasks. However, for more than simple tasks, such spells are a bugger to cast to a fine degree of accuracy. You can throw a boulder in some haphazard direction with relative ease, but taking said boulder and placing it squarely on the peak of a mountain is something most mages don't even bother trying to do for both practical reasons and sheer lack of magical stamina to get the job done without dropping the boulder.

Alex is using such a spell to push away the muck that is has caused her foot to become stuck in the swamp. Since muck is definately not all that hard of a thing to move, the spell is not too hard to cast, allowing her enough spare concentration to maintain a deadpan attitude as she talks to Matt. However, one may also wonder why she bothered to cast the spell in the first place. It would of course be less magically taxing on Alex if she were to simply keep trying to pull her foot until it popped out. It may be implied that this is a waste of magic, that she is simply a bit lazy to do it the manual way, or she's just showing off to nobody in particular. This, however, would be a bit harsh of an assumption. One COULD pull one's foot out of the muck, especially this near the edge of the swamp, where the muck is shallower. However, this comes with the unfortunate side effect of certain laws of physics which govern issues regarding actions causing equal and opposite reactions. Specifically, one would be thrown backward by the force of the suddenly free leg, causing one to fall backward into said muck. The consequences of such an action are obvious.

Plus, one would stand a good chance of losing one's footwear.