Why does my yellow underglaze go dull under clear glaze?

That muddiness is pretty normal, and it's usually the glaze, not you. A lot of commercial clears have opacifiers or a slight milky quality in the melt, and yellow underglazes are the first thing to suffer from that. Reds too, since they tend to be less saturated to begin with. Redoing the application thinner helps sometimes, but if the glaze itself is the culprit you'll just get thinner muddiness. Two things I'd try on test tiles before you commit this piece. First, check whether your clear is actually transparent when thin. Apply one thin coat, or dip for a couple seconds instead of the usual, and see if the yellow pops more. Some clears clear up beautifully at two thin coats and go foggy at three. If thinning fixes it, you're done, no third firing needed.

If it still dulls everything even when applied properly, then yes, the post-glaze color route is worth it. What I've done is glaze fire the piece plain, then paint the underglaze color right on top of the fired glaze and refire to a low bisque-ish temperature. The colors sit on the surface so they stay bright and saturated, way punchier than under a clear. The tradeoff is they come out matte, so on a piece like yours I'll add a thin brush coat of clear over just the colored bits and fire again. That's technically a fourth firing but it's a quick low one.

One warning on that approach: underglaze over a smooth fired glaze can chip or flake at edges since it's just sitting on top, so keep it in the carved areas and flat zones where it can grip, not on high-touch rims. Your sunbeam channels are actually ideal for it since the carving gives it some tooth. Also worth checking what cone you're firing the clear to. If you're pushing it hot, some yellows break down and gray out in the melt itself, no glaze needed for that. Dropping the clear fire a cone or two sometimes solves it on its own.

Yeah, the color on top of fired glaze trick is real. I did something similar on a fish platter last year, painted the scales over the fired surface and refired low, and the color came out so much brighter than my earlier test where the clear went over everything. I brushed clear on just the scales and it stayed put fine since the design was all in shallow carved lines. Same as said above about edges, I kept it away from the rim and had zero flaking.

This reminds me of how the old studio folks I learned from handled sgraffito, they mostly skipped clear altogether. Rub the color into the carved lines while the piece is still raw, buff the surface back with a sponge, fire once, and let the slip do the talking.Buffing the slip down before it leathered gave them a soft sheen that read plenty bright without anything over it. Different look than a glassy surface, more matte and earthy, but those yellows never went muddy because nothing sat on top of them.Worth considering if you ever get tired of chasing the clear. On a piece like yours the unglazed carved rays might actually suit the subject.

A lot of people focus only on the clear glaze, but sometimes the real difference is in the surface prep before you even fire. On a piece I did last year, the final look was totally transformed by just switching from a regular sea sponge to a fine piece of pumice when I buffed the raised buff stoneware after the slip dried. It gave the surface a subtle textural variation that caught the light differently than the smooth matte finish.

It really makes the carved lines of the black slip pop because the contrast is textural, not just color-based. Have you considered experimenting with diffrent textures on the surrounding buff clay? It might help enhance the depth of the carving even if the clear still comes out a bit muddy.