"Strong enough" is a relative term and depends on "for what?". If comparing the relative strength of 6061 aluminum to HDPE, the plastic would need to be 2-1/2 times thicker than the aluminum to offer the same strength. This is from a site discussing boat hull strength:
Hull material
That said, I don't ever plan to slide over rocks, or head out to sea in my sxs, so either material would be fine (my SE came with an aluminum skid plate that I don't think I'll be changing). However, if starting from the get-go on a DIY project to make a skid plate, at least in terms of strength (impact, penetration, resistance to abrasion etc.), and if money wasn't too much of an object for the materials being considered, it would be UHMW, aluminum plate, and HDPE in that order. The differences in the relative strength between those materials is a whole bunch!
6061 vs. HDPE
The stock aluminum Honda skid plate is fine, offering reasonable protection, though not for sliding over rocks regular like. UHMW is a whole lot better, stronger in every respect, and much more slippery. Of the materials known to man, UHMW is by far the best suited for this purpose.