The example doesn't work. Am I doing something wrong ?

[schau87]

$ mibdump.py --generate-mib-texts --destination-format json IF-MIB
Source MIB repositories: file:///usr/share/snmp/mibs, http://mibs.snmplabs.com/asn1/@mib@
Borrow missing/failed MIBs from: http://mibs.snmplabs.com/json/fulltexts/@mib@
Existing/compiled MIB locations:
Compiled MIBs destination directory: .
MIBs excluded from code generation: RFC-1212, RFC-1215, RFC1065-SMI, RFC1155-SMI, RFC1158-MIB, RFC1213-MIB, SNMPv2-CONF, SNMPv2-SMI, SNMPv2-TC, SNMPv2-TM
MIBs to compile: IF-MIB
Destination format: json
Parser grammar cache directory: not used
Also compile all relevant MIBs: yes
Rebuild MIBs regardless of age: no
Dry run mode: no
Create/update MIBs: yes
Byte-compile Python modules: no (optimization level no)
Ignore compilation errors: no
Generate OID->MIB index: no
Generate texts in MIBs: yes
Keep original texts layout: no
Try various file names while searching for MIB module: yes
Created/updated MIBs:
Pre-compiled MIBs borrowed:
Up to date MIBs:
Missing source MIBs: IF-MIB
Ignored MIBs:
Failed MIBs:

[lextm]

You got "Missing source MIBs: IF-MIB" simply because http://mibs.snmplabs.com is down.

As part of etingof/pysnmp#429, my company set up https://mibs.pysnmp.com to replace that lost site.

You can simply switch to pysmi-lextudio package, which by default uses the new MIBs site.