If you are trying to get a regional job and don't have competitive time take what you can get.
If you are competitive look for a regional that will treat you well. You may be there for 5 - 10 years or more as an FO. If Mesa is the only one offering, take it.
Later on, after you have had time to talk to other pilots from the various regionals and guage them, you can move on from Mesa.
I switched from Trans States to ACA and have no regrets. ACA has treated me well, TSA had good people but crappy owners(HK). The downside to working for sub par companies is the increased chance of getting violated or the company hanging you out to dry with the FEDs.
One of my Captains at TSA had the chief pilot go after him, accusing him of striking a taxi light with a prop. It was a bird strike, and the airport showed him the bird. He was eventually cleared but it was an ordeal, the union saved his butt. You get the idea.
Good Luck,