Yeah, Spaniards pronouncing a lot of 's' and 'c' sounds as English 'th' or 'z' is called the ceceo, and it's commonly incorrectly attributed to Spaniards copying a prior king's lisp.
If you drive from Virginia to Boston, you'll probably hear 5 different pronunciations of "Grassy ass". I don't think any of them sound like how I learned to say "thank you" in high school Spanish class.
Bot is a problematic example, as it has many different pronunciations depending on meaning and region. But non on the common pronunciations have an 'a' (IPA), like Gracias.
True, I was simplifying a bit. Technically "gracias" is /a/ as you point out, while bot is /ɑ/. But /a/ doesn't really exist in General American English, so bot (or bra, as sibling suggests) is the closest on the IPA vowel chart.
What's wrong with it? I'm a native speaker of neither, but they do sound similar (of course, gracias being pronounced South American style, not Spanish style). Except the fact that Spanish and English "r" differ, but I think it's reasonable for English speakers to know that, once they learn to pronounce Spanish "r".
