There's definitely a difference between a conservative tax return and an aggressive tax return.
Lots of tax laws can be considered grey areas with the right accountant. As long as you keep good records that's what counts
Question, did you pay any taxes on your mystery shopping income last year? Did you get any 1099s. If you did, the hotel costs would offset that taxed income as an expense. If you didn't, he was probably right.
I'm not an accountant. But someone here could probably tell you if a loss in your business could reduce your taxable income from a "real" job.
It might be your preparer was lazy, uneducated on all the tax laws or perhaps figured doing all the additional work would have cost you more billable hours than any gains.