Privacy

One thing to keep in mind as new is that "VPN" is a technical term with pretty clear meaning among the technical people but it has a very fuzzy meaning in marketing and branding. Referring here to "VPN apps" that may just be a local DNS relay (ie: it will only tunnel and filter your DNS requests; all your actual traffic still goes through your normal connection as clear as always). Oftentimes, it's what we would call a proxy. Android has not at all helped here.

In either case, yes, you can usually chain things. What if any benefits you get from that depends on both technical specifics (which protocols) and your circumstances and threat model.

For example, if we consider only Wireguard (one of the VPN protocols Mullvad offers).

No VPN/proxy: Your ISP sees everything

1 proxy: ISP sees that you are connecting to proxy but not what servers you're actually talking to. VPN provider now sees everything instead.

2 proxies: Proxy A sees your encrypted traffic to Proxy B. Proxy B sees all your traffic but doesn't know where you are.

3 proxies: Congratulations, you have manually built a shitty onion circuit (Tor works like this)

Mullvad has their own "multi-hop" feature which chains two Mullvad nodes but i have to question using that strictly for privacy reasons, considering it's by the same provider and the ports make it predictable from the ISP.