How to deal with "Brown on" slowly increasing voltage/current from solar panel to power mcu
The SAMD21 contains a power-on-reset (POR) and brown-out detector (BOD) circuitry. Depending on the settings they may not be in use by default, so you may have to specifically enable them. Otherwise you have to use an external voltage supervisor IC to keep the SAMD21 in reset until supply voltage is high enough.

Note that the SAMD21 can work down to 1.62 V supply voltage, but it certainly can't work at maximum speed with minimum voltage, so you need to check either how large supply voltage you need for your current clock speed, or how fast the clock can be with the given supply voltage you have.