I think the other posters have explained that a thickening crest can be a *sign* of overweight and tendency to to founder, not a conformation flaw that indicates a propensity to founder.
Also, if you're managing a pony prone to founder, it's a good idea to monitor the crest of their neck on a daily basis, as well as their stance and the condition of their feet.
With my 10 hand pony, I palpate the top of his neck for the size of the fat deposits; and wiggle it back and forth to see how tense and rigid it is. I then look at his back and rump to see how much of a negative crease there is along the spine, and if there are fat deposits at the tail head.
This "exam" is cleverly disguised as petting and fussing over him, and can be done in seconds.
Finally, I look at his stance - if he has his front legs out in front of him and is rocked back, avoiding putting weight on his heels, that's a very late and serious sign of imminent founder. Ditto if his hooves are warm to the touch, or if he has a throbbing digital pulse.
As soon as his neck gets tense, he starts wearing a grazing muzzle. If that doesn't bring him in line within a few days, he spends some time in a stall or dry lot or we add some forced exercise - lunging or driving or trotting beside the golf cart. If you're doing a good job managing him, you should never see the founder stance or have warm hooves or palpable pulses.
HTH, enjoy your little guy.