Big voltage drop across MOSFET with LED strip load
You need a logic-level MOSFET rated for &lt;= 3.3V gate drive to make that work. You have a MOSFET rated for 4V gate drive according to the datasheet so you're not completely driving it 'on'. 

It's a bit surprising it's that bad, if the part is genuine. Measure Vgs directly at the MOSFET, from gate to source. If there is excessive resistance in the source circuit it could be taking away the already inadequate Vgs voltage.