I know we think Becky should have been cleaning out Amberlynn's navel all along, and maybe she should have, but Becky also thinks "marjoram" is an old-fashioned way of saying "margarine" and the only reason she didn't shoot herself is because someone shot at a possum or some other varmint and left her no ammunition. When she ran away she planned to bathe in a stream every morning and go to high school like normal. She's not going to be intelligent enough to research online how to cope with elements of bariatric hygiene.
Amber's navel probably has a depth of around four to six inches at this point. Does Becky even know extra-long cotton swabs exist? I don't even know how Becky would be able to problem solve this, even with the entirety of the Internet at her disposal. She'd probably ask Eric for help, who would then suggest something hilariously awful, like wrapping wet paper towels around a butter knife and swirling it until Amber hollered. It's best in the long run for her to stay away from Amber's grosser parts and haul her away to let professionals deal with bleeding infections when you're a Becky like our Becky.
Also no way they're having sex. No way either of them have any libido at their current sizes. When someone gets this fat, the only way sex happens is if the fat partner starfishes and lets the fit partner do what they need to do. So it's not like Becky would have been alarmed by any new odor. Amber smells but Becky's not getting in close enough to realize a new horrible odor is on the scene.