This issue has been discussed or touched on here some, for example: A bisq-like dex to trade ERG for fiat or BTC and other cryptcurrencies
or Ergo Escrow: Craigslist improvement for real-fi.
But in terms of specifically a sigusd on ramp, rather that ERG or other tokens, I would say that the friction is likely to be higher than credit cards if the goal is to facilitate payments. And if the goal is to stimulate defi, then I cannot see any advantage over onboarding with Erg itself, since that is the most liquid trading pair for everything, including SigUSD.
So if I want, say $PAI, and I only have fiat, it would be cheaper to buy ERG and trade that to PAI rather than making the switch via SigUSD. Also easier, but that is less of an issue than cheaper.
Another problem is that the stability mechanism for SigUSD requires oscillating between 400-800% treasury for backing, so if your idea took hold, there would need to be a larger pool of SigUSD available for trade, which would first need to be minted (a cost), and then balanced with the minting of SigRSV (another cost), and also the opportunity cost of tying up all that ERG.
There are other stable coins coming that could be more useful for the purpose, especially Chain Cash: ChainCash - a spender-signed currency on Ergo
In any case, the issue of on ramps is complicated by regulations and by the tendency to rely on central exchanges. Right now the most important thing that you could choose to buy with SigUSD is probably ERG, and after the bull run heats up, the best thing to buy with ERG is SigUSD. It is primarily a tool for hedging and taking profits, while remaining in the ecosystem (to trade back to ERG during the next bear market).
Those are my thoughts, hopefully they help.