There are a couple confusions at work here. First, there are different "lexical sets" the word could fall into: TRAP, LOT, BATH, CLOTH, PALM, THOUGHT, NORTH, FORCE.
For the most part, most speakers and dictionaries of English agree that mall is either pronounced with the vowel for THOUGHT or with the vowel for TRAP, the TRAP pronunciation usually only in the context of pall-mall.
No dialect pronounces all those lexical sets with distinct vowels, but each dialect may merge them in different ways. American English generally merges TRAP-BATH, LOT-PALM, CLOTH-THOUGHT, and NORTH-FORCE. This means that Americans use the same vowel for TRAP as for BATH, the same vowel for LOT as for PALM, and the same vowel for CLOTH as for THOUGHT. British English generally merges LOT-CLOTH, BATH-PALM, THOUGHT-NORTH-FORCE, and keeping TRAP in a distinct group. Furthermore, the cot-caught merger in some varieties of American English merge the LOT-PALM group with the CLOTH-THOUGHT group, making a single lexical set LOT-PALM-CLOTH-THOUGHT, all pronounced with the same vowel (/ɑ/)—for some speakers of American English (me included).
So, a British English-speaking person would think they might hear me say mall thinking it sounds like moll (that is, to be in the LOT set), and that I am pronouncing it "wrong", but not realize that I pronounce all LOT words with the same vowel as THOUGHT words—that is, for me the words LOT and THOUGHT rhyme.
In conclusion, the vowel of mall should be the same as the vowel for thought, however you pronounce it in your dialect.