You think so, eh? Think about it. How do sans' attacks work? 1 dmg + heavy poison effect… but they hit once every frame (I'm assuming, and I think the game runs at 30 fps? I remember hearing Everdraed mentioning 42 fps but I think that was something else) so that's 30 DPS + the poison. Now… considering sans' attacks… dozens of bones and gasterblasters all attacking at once… and, given Omega Flowey's size, he's not going to be hit by only one at a time. Flowey can't dodge, either. sans can.

Also, a bonus theory: what if the reason sans is fighting Omega Flowey is because Frisk died… but sans got his SOUL? Then sans would have the SOUL that Flowey needs, and sans would probably step up… probably.

Odds are, Frisk's soul could resonate (and provide help) from within Sans much like the other human souls from within Flowey, so the same events could theoretically transpire for them to turn on him. Frisk was able to defeat him alone with their help. Frisk-Sans would be even stronger, and he definitely no longer be "the weakest enemy in the game." Even Chara-Asriel had godlike power with just those two souls, and it's likely Asriel himself wasn't strong at all yet (I think it was said that boss monsters grow and gain power as their parents age). But Sans, offensively, probably the strongest in the game combined with Frisk's ability to power through everything thrown at 'em… yeah. Gonna have a good time.
…
WAIT. Sans would have *Frisk's* SOUL. Frisk had the power to SAVE which usurped Flowey's, and apparently what the other humans lacked as well (cuz they're dead). But now, in this hypothetical… wouldn't Sans then inherit that ability from Frisk, except escalated to a demigod-like level due to the monster-human soul combo. Sans already seems well aware of how the timeline jumps around due to the saving and loading, too, and seems to walk through space-time itself or something. Potentially, Frisk-Sans could supersede even Omega Flowey's ability to SAVE.